diff --git a/lib/iris/tests/__init__.py b/lib/iris/tests/__init__.py
index 15a3fada31..9e46dd913e 100644
--- a/lib/iris/tests/__init__.py
+++ b/lib/iris/tests/__init__.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2017, Met Office
+# (C) British Crown Copyright 2010 - 2018, Met Office
 #
 # This file is part of Iris.
 #
@@ -552,7 +552,6 @@ def assertWarnsRegexp(self, expected_regexp=''):
         msg = msg.format(expected_regexp)
         self.assertTrue(matches, msg)
 
-
     @contextlib.contextmanager
     def assertNoWarningsRegexp(self, expected_regexp=''):
         # Check that no warning matching the given expression is raised.
@@ -782,7 +781,7 @@ def check_graphic(self):
             def _create_missing():
                 fname = '{}.png'.format(phash)
                 base_uri = ('https://scitools.github.io/test-iris-imagehash/'
-                            'images/{}')
+                            'images/v4/{}')
                 uri = base_uri.format(fname)
                 hash_fname = os.path.join(image_output_directory, fname)
                 uris = repo.setdefault(unique_id, [])
@@ -817,13 +816,14 @@ def _create_missing():
                     raise AssertionError(emsg.format(unique_id))
             else:
                 uris = repo[unique_id]
+                # Extract the hex basename strings from the uris.
+                hexes = [os.path.splitext(os.path.basename(uri))[0]
+                         for uri in uris]
                 # Create the expected perceptual image hashes from the uris.
                 to_hash = imagehash.hex_to_hash
-                expected = [to_hash(os.path.splitext(os.path.basename(uri))[0],
-                                    hash_size=_HASH_SIZE)
-                            for uri in uris]
+                expected = [to_hash(uri_hex) for uri_hex in hexes]
 
-                # Calculate the hamming distance vector for the result hash.
+                # Calculate hamming distance vector for the result hash.
                 distances = [e - phash for e in expected]
 
                 if np.all([hd > _HAMMING_DISTANCE for hd in distances]):
@@ -1104,7 +1104,7 @@ def assertGribMessageDifference(self, filename1, filename2, diffs,
                         # for each message.
                         self.assertEqual(m1_value, diffs[key][0],
                                          msg=msg.format(key, m1_value,
-                                                         diffs[key][0]))
+                                                        diffs[key][0]))
 
                         self.assertEqual(m2_value, diffs[key][1],
                                          msg=msg.format(key, m2_value,
diff --git a/lib/iris/tests/idiff.py b/lib/iris/tests/idiff.py
index 919947196f..2267375928 100755
--- a/lib/iris/tests/idiff.py
+++ b/lib/iris/tests/idiff.py
@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-# (C) British Crown Copyright 2010 - 2017, Met Office
+# (C) British Crown Copyright 2010 - 2018, Met Office
 #
 # This file is part of Iris.
 #
@@ -83,7 +83,7 @@ def diff_viewer(repo, key, repo_fname, phash, status,
 
     result_dir = os.path.dirname(result_fname)
     fname = '{}.png'.format(phash)
-    base_uri = 'https://scitools.github.io/test-iris-imagehash/images/{}'
+    base_uri = 'https://scitools.github.io/test-iris-imagehash/images/v4/{}'
     uri = base_uri.format(fname)
     phash_fname = os.path.join(result_dir, fname)
 
@@ -139,24 +139,26 @@ def skip(event):
 
 
 def _calculate_hit(uris, phash, action):
+    # Extract the hex basename strings from the uris.
+    hexes = [os.path.splitext(os.path.basename(uri))[0] for uri in uris]
     # Create the expected perceptual image hashes from the uris.
     to_hash = imagehash.hex_to_hash
-    expected = [to_hash(os.path.splitext(os.path.basename(uri))[0],
-                        hash_size=iris.tests._HASH_SIZE)
-                for uri in uris]
+    expected = [to_hash(uri_hex) for uri_hex in hexes]
     # Calculate the hamming distance vector for the result hash.
     distances = [e - phash for e in expected]
+
     if action == 'first':
         index = 0
     elif action == 'last':
         index = -1
     elif action == 'similar':
         index = np.argmin(distances)
-    elif action == 'difference':
+    elif action == 'different':
         index = np.argmax(distances)
     else:
         emsg = 'Unknown action: {!r}'
         raise ValueError(emsg.format(action))
+
     return index, distances[index]
 
 
@@ -230,7 +232,7 @@ def step_over_diffs(result_dir, action, display=True):
                                                os.path.basename(uri))
                     if not os.path.isfile(local_fname):
                         emsg = 'Bad URI {!r} for test {!r}.'
-                        raise ValueError(uri, key)
+                        raise ValueError(emsg.format(uri, key))
                     else:
                         # The temporary expected filename has the test name
                         # baked into it, and is used in the diff plot title.
diff --git a/lib/iris/tests/results/imagerepo.json b/lib/iris/tests/results/imagerepo.json
index 60d5a961fe..90c888c7c9 100644
--- a/lib/iris/tests/results/imagerepo.json
+++ b/lib/iris/tests/results/imagerepo.json
@@ -1,725 +1,726 @@
 {
     "example_tests.test_COP_1d_plot.TestCOP1DPlot.test_COP_1d_plot.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/5dff1a996c06b47193eecc52275b936d58b6239ba94c133643897c6c2333f086.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/baff589936602d8ec977334ae4dac9b61a6dc4d99532c86cc2913e36c4cc0f61.png"
+    ], 
     "example_tests.test_COP_maps.TestCOPMaps.test_cop_maps.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/57891cdba966a124897c568316997ea1a97e897ee1c6699d5681ad733c9296ac.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/ea9138db95668524913e6ac168997e85957e917e876396b96a81b5ce3c496935.png"
+    ], 
     "example_tests.test_SOI_filtering.TestSOIFiltering.test_soi_filtering.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/5f23069d83de1e4e7ca0a595a9725b0f46cce499a97239a563dc594a4b725aa9.png",
-        "https://scitools.github.io/test-iris-imagehash/images/5f21069d83de1e4e7ca0a595a9725b0f46ccec99a97239a563dc594a4b7252b9.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fac460b9c17b78723e05a5a9954edaf062332799954e9ca5c63b9a52d24e5a95.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fa8460b9c17b78723e05a5a9954edaf062333799954e9ca5c63b9a52d24e4a9d.png"
+    ], 
     "example_tests.test_TEC.TestTEC.test_TEC.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/87a5866dd958594221765992e39a763c733609de5cc1837ed8419ccd27cfdc23.png",
-        "https://scitools.github.io/test-iris-imagehash/images/87a5866dd95879c221765992e39a7634733609de5cc18376d8498ccd27cfdc31.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e1a561b69b1a9a42846e9a49c7596e3cce6c907b3a83c17e1b8239b3e4f33bc4.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e1a561b69b1a9e43846e9a49c7596e2cce6c907b3a83c16e1b9231b3e4f33b8c.png"
+    ], 
     "example_tests.test_anomaly_log_colouring.TestAnomalyLogColouring.test_anomaly_log_colouring.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/37222687a1c5f9c9c978d9788996b69492bb67677c64255e5acb89c9b1595a30.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/ec4464e185a39f93931e9b1e91696d2949dde6e63e26a47a5ad391938d9a5a0c.png"
+    ], 
     "example_tests.test_atlantic_profiles.TestAtlanticProfiles.test_atlantic_profiles.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/f94106cad64b71c804ce29ecad2fec0da5b8662f33ba103f0bf0db38c93f4d38.png",
-        "https://scitools.github.io/test-iris-imagehash/images/f94106cad64b71c804ce29ecad2fec0da5b8662f333e902f0bf0db38c93f4d38.png",
-        "https://scitools.github.io/test-iris-imagehash/images/f95106cad64b71c804ce29ecad2fec0da5bc662f333a102f0bf0db38c93f4d38.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/9f8260536bd28e1320739437b5f437b0a51d66f4cc5d08fcd00fdb1c93fcb21c.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/9f8260536bd28e1320739437b5f437b0a51d66f4cc7c09f4d00fdb1c93fcb21c.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/9f8a60536bd28e1320739437b5f437b0a53d66f4cc5c08f4d00fdb1c93fcb21c.png"
+    ], 
     "example_tests.test_atlantic_profiles.TestAtlanticProfiles.test_atlantic_profiles.1": [
-        "https://scitools.github.io/test-iris-imagehash/images/6557a57e7681bb9f998cd8c5cdee7a0421ba1a918399e6dc724425e67a318538.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/a6eaa57e6e81ddf999311ba3b3775e20845d5889c199673b4e22a4675e8ca11c.png"
+    ], 
     "example_tests.test_coriolis_plot.TestCoriolisPlot.test_coriolis_plot.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/e761a67b5996699aa77a99a611969e699661a3677c1983795ca4669e87195824.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e78665de9a699659e55e9965886979966986c5e63e98c19e3a256679e1981a24.png"
+    ], 
     "example_tests.test_cross_section.TestCrossSection.test_cross_section.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/57a98cdea946278b26f95aa0a17632255b4a297e72a58ffcd892b9428fdcd882.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/ea95317b9562e4d1649f5a05856e4ca4da52947e4ea5f13f1b499d42f13b1b41.png"
+    ], 
     "example_tests.test_cross_section.TestCrossSection.test_cross_section.1": [
-        "https://scitools.github.io/test-iris-imagehash/images/57a984dfa9569c02964978c90ffe52b5a136237e72a9a15e78a55bac895dd881.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/ea9521fb956a394069921e93f07f4aad856cc47e4e95857a1ea5da3591ba1b81.png"
+    ], 
     "example_tests.test_custom_aggregation.TestCustomAggregation.test_custom_aggregation.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/7f817681897e097e2d7ce1fca1e65e83090f0e3c56a981f858c33c87a55ef618.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fe816e81917e907eb43e873f85677ac190f0703c6a95811f1ac33ce1a57a6f18.png"
+    ], 
     "example_tests.test_custom_file_loading.TestCustomFileLoading.test_custom_file_loading.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/5f05d38f217a2c7d89ece182763b133ddc13f8d9c6cc644625b70cb3834db384.png",
-        "https://scitools.github.io/test-iris-imagehash/images/df05d38f217a2c7d89e4e182763b133ddc11f8d946cce446a5b54cb3834db384.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/faa0cbf1845e34be913787416edcc8bc3bc81f9b63332662a4ed30cdc1b2cd21.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fba0cbf1845e34be912787416edcc8bc3b881f9b62332762a5ad32cdc1b2cd21.png"
+    ], 
     "example_tests.test_deriving_phenomena.TestDerivingPhenomena.test_deriving_phenomena.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/9d999c6161964a679362629c236ed69b63968976de998366fc996e91096e7c81.png",
-        "https://scitools.github.io/test-iris-imagehash/images/9d899c7b61964a679362639c237ed68be1968b765e99816674816c990b6e7c91.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/b9993986866952e6c9464639c4766bd9c669916e7b99c1663f99768990763e81.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/b99139de866952e6c946c639c47e6bd18769d16e7a9981662e813699d0763e89.png"
+    ], 
     "example_tests.test_global_map.TestGlobalMap.test_global_map.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/5f999e62a166a17e0f7e7c911e6ad689d3809e113c9129666195d6b9c16ef681.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fa9979468566857ef07e3e8978566b91cb0179883c89946686a96b9d83766f81.png"
+    ], 
     "example_tests.test_hovmoller.TestGlobalMap.test_hovmoller.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/5d2d0c2d73d273c2a37df391a3d258c6a3c2a3767826097edc2d962d097b5883.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/bab430b4ce4bce43c5becf89c54b1a63c543c56e1e64907e3bb469b490de1ac1.png"
+    ], 
     "example_tests.test_inset_plot.TestInsetPlot.test_inset_plot.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/d7ff9649af0069a54da25b236faa29692d4912db93bad396a99489b4f324522a.png",
-        "https://scitools.github.io/test-iris-imagehash/images/97ff9649ad0069a54da25b236fa2296d2d4912db93bad396a994a9b4f324526a.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/ebff6992f50096a5b245dac4f6559496b49248dbc95dcb699529912dcf244a54.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e9ff6992b50096a5b245dac4f64594b6b49248dbc95dcb699529952dcf244a56.png"
+    ], 
     "example_tests.test_lagged_ensemble.TestLaggedEnsemble.test_lagged_ensemble.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/dddd8c87237226270da2d9da8d8e765323262f69732c86718de0d99c8dc973a4.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/bbbb31e1c44e64e4b0459b5bb1716ecac464f496ce34618eb1079b39b193ce25.png"
+    ], 
     "example_tests.test_lagged_ensemble.TestLaggedEnsemble.test_lagged_ensemble.1": [
-        "https://scitools.github.io/test-iris-imagehash/images/d57f9f1abf6234995ce01b9e0662d2818b0069e1839ccbad2997f3e1631d69e1.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/abfef958fd462c993a07d87960464b81d1009687c139d3b594e9cf87c6b89687.png"
+    ], 
     "example_tests.test_lineplot_with_legend.TestLineplotWithLegend.test_lineplot_with_legend.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/5797424aa6021d9669f8b165291ab965a9c233598f8052dfc3bf9ad6217f98e5.png",
-        "https://scitools.github.io/test-iris-imagehash/images/57974228a6021d9669f8b167291ab965a9c233598f8052dfc3bf9ad6217f98e5.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/eae942526540b869961f8da694589da69543cc9af1014afbc3fd596b84fe19a7.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/eae942146540b869961f8de694589da69543cc9af1014afbc3fd596b84fe19a7.png"
+    ], 
     "example_tests.test_orca_projection.TestOrcaProjection.test_orca_projection.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/df88ce582973257726cd7a898b4b0c72795ae39cde04877f48a124e167667362.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fb11731a94cea4ee64b35e91d1d2304e9e5ac7397b20e1fe12852487e666ce46.png"
+    ], 
     "example_tests.test_orca_projection.TestOrcaProjection.test_orca_projection.1": [
-        "https://scitools.github.io/test-iris-imagehash/images/a765a665599a699aa7db18a643a6dc61996933677c9987595889649ce71878a6.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e5a665a69a599659e5db1865c2653b869996cce63e99e19a1a912639e7181e65.png"
+    ], 
     "example_tests.test_orca_projection.TestOrcaProjection.test_orca_projection.2": [
-        "https://scitools.github.io/test-iris-imagehash/images/4f232673799cc94c87ed3287339ecc31a661a7cddc8ccd5e6693663203765826.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/f2c464ce9e399332e1b74ce1cc79338c6586e5b33b31b37a66c9664cc06e1a64.png"
+    ], 
     "example_tests.test_orca_projection.TestOrcaProjection.test_orca_projection.3": [
-        "https://scitools.github.io/test-iris-imagehash/images/5f815ec121762536a79c93cc897b4c3361f3e1c5fc85164e38db7c9176ecd220.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fa817a83846ea46ce539c93391de32cc86cf87a33fa168721cdb3e896e374b04.png"
+    ], 
     "example_tests.test_polar_stereo.TestPolarStereo.test_polar_stereo.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/87168c5e49cbb691a3dd7929a37af63059c9835a76a3216edc848ee6897b5c81.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e168317a92d36d89c5bb9e94c55e6f0c9a93c15a6ec584763b21716791de3a81.png"
+    ], 
     "example_tests.test_polynomial_fit.TestPolynomialFit.test_polynomial_fit.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/d5ff52b94f26ac11a60493fe488262a9236db9c4c9d213563b6949ec6b3133f6.png",
-        "https://scitools.github.io/test-iris-imagehash/images/55ff52b94f26ac11a60493fe488262a9236db9c4c9d213563b6959e86b3933f6.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/abff4a9df26435886520c97f12414695c4b69d23934bc86adc969237d68ccc6f.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/aaff4a9df26435886520c97f12414695c4b69d23934bc86adc969a17d69ccc6f.png"
+    ], 
     "example_tests.test_projections_and_annotations.TestProjectionsAndAnnotations.test_projections_and_annotations.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/5fa1f298a1580c27336eb3d08d9e4c3ae563a60d931cd3d2728e7939ede4ad03.png",
-        "https://scitools.github.io/test-iris-imagehash/images/5fa3f298a1580c27336eb3d08d9e4c3aed61a60d931cd3d2728e7939e9e4ad03.png",
-        "https://scitools.github.io/test-iris-imagehash/images/5fa17298a1580c27336eb3d08d9e4c3aed63260d931cd3d273ce7939ecc5ad03.png",
-        "https://scitools.github.io/test-iris-imagehash/images/5fa17298a1580c27336eb3d08d9e4c3aed61a68d931c93d273ce7939ece4ad03.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fa854f19851a30e4cc76cd0bb179325ca7c665b0c938cb4b4e719e9cb727b5c0.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fac54f19851a30e4cc76cd0bb179325cb78665b0c938cb4b4e719e9c9727b5c0.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fa854e19851a30e4cc76cd0bb179325cb7c664b0c938cb4bce739e9c37a3b5c0.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/fa854e19851a30e4cc76cd0bb179325cb78665b1c938c94bce739e9c3727b5c0.png"
+    ], 
     "example_tests.test_projections_and_annotations.TestProjectionsAndAnnotations.test_projections_and_annotations.1": [
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     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_zx.2": [
-        "https://scitools.github.io/test-iris-imagehash/images/e1b7f461fe00b14104e8194a0ff89b7d8b1e5936213e13ee2356934e197e13bf.png",
-        "https://scitools.github.io/test-iris-imagehash/images/e1b7f460fe00b14104e8194a0ff89b7d8b1e5936213e13ee2316936f197e13bf.png",
-        "https://scitools.github.io/test-iris-imagehash/images/e1b7f460fe00b14104e8394a0ff89b7d8b1e5936213e13eea306936e197e13bf.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/87ed2f867f008d8220179852f01fd9bed1789a6c847cc877c46ac972987ec8fd.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/87ed2f067f008d8220179852f01fd9bed1789a6c847cc877c468c9f6987ec8fd.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/87ed2f067f008d8220179c52f01fd9bed1789a6c847cc877c560c976987ec8fd.png"
+    ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_zx.3": [
-        "https://scitools.github.io/test-iris-imagehash/images/459fad5d6e00a59646fb79690fb893642319336221feb9360b24d28f59d6988f.png",
-        "https://scitools.github.io/test-iris-imagehash/images/459fad5d6e00a19646fb79690fb8b3642319336221feb9360b24d28f59d698ae.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/a2f9b5ba7600a56962df9e96f01dc926c498cc46847f9d6cd0244bf19a6b19f1.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/a2f9b5ba7600856962df9e96f01dcd26c498cc46847f9d6cd0244bf19a6b1975.png"
+    ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_zx.4": [
-        "https://scitools.github.io/test-iris-imagehash/images/75a9fce1ab17b46101f8897776a8897f7e881e6e03be16ee0b161b1e1b000b5e.png",
-        "https://scitools.github.io/test-iris-imagehash/images/75a9fce1ab17b4e101d8897776a8997f7e881e2e03be16ee0b161b1e1b000b5e.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/ae953f87d5e82d86801f91ee6e1591fe7e117876c07d6877d068d878d800d07a.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/ae953f87d5e82d87801b91ee6e1599fe7e117874c07d6877d068d878d800d07a.png"
+    ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_zx.5": [
-        "https://scitools.github.io/test-iris-imagehash/images/175ea9b469a196c269f9966269a1b172078ab97607fed9561e86d9549e8e5854.png",
-        "https://scitools.github.io/test-iris-imagehash/images/175ea9b469a196c269f9966269a1b152078ab97607fe99561e8ed9549e8e585c.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e87a952d96856943969f694696858d4ee0519d6ee07f9b6a78619b2a79711a2a.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e87a952d96856943969f694696858d4ae0519d6ee07f996a78719b2a79711a3a.png"
+    ], 
     "iris.tests.test_quickplot.TestTimeReferenceUnitsLabels.test_not_reference_time_units.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/415f85e9fefb91e94600bb6f07009be7effa1966ab065b273b009b663b007a04.png",
-        "https://scitools.github.io/test-iris-imagehash/images/411d85e9fefb91e14600bb6707009be7effe1966ab06fb273b009b663f007a04.png",
-        "https://scitools.github.io/test-iris-imagehash/images/411f85e9fefb91e14600bb6f07009be7effe1966ab067b273b009b663b007a04.png",
-        "https://scitools.github.io/test-iris-imagehash/images/411f85e9fefb91e14600bb6707009be7effe1966ab06fb273b00bb263b007a04.png"
-    ],
+        "https://scitools.github.io/test-iris-imagehash/images/v4/82faa1977fdf89976200ddf6e000d9e7f75f9866d560dae4dc00d966dc005e20.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/82b8a1977fdf89876200dde6e000d9e7f77f9866d560dfe4dc00d966fc005e20.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/82f8a1977fdf89876200ddf6e000d9e7f77f9866d560dee4dc00d966dc005e20.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/82f8a1977fdf89876200dde6e000d9e7f77f9866d560dfe4dc00dd64dc005e20.png"
+    ], 
     "iris.tests.test_quickplot.TestTimeReferenceUnitsLabels.test_reference_time_units.0": [
-        "https://scitools.github.io/test-iris-imagehash/images/417f8119feebeeff070054bb2b0014a0bb157ba6bb972b46dabf3b0419827b04.png",
-        "https://scitools.github.io/test-iris-imagehash/images/417f8119fefbeeff070054b92b0014a0bb557ba69b95ab46dabf3b0419827b04.png",
-        "https://scitools.github.io/test-iris-imagehash/images/417f8119fefbeeff070054bb2b0014a0bb14fbe69b952b46dabf3b0419827b04.png"
+        "https://scitools.github.io/test-iris-imagehash/images/v4/82fe81987fd777ffe0002addd4002805dda8de65dde9d4625bfddc209841de20.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/82fe81987fdf77ffe0002a9dd4002805ddaade65d9a9d5625bfddc209841de20.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/v4/82fe81987fdf77ffe0002addd4002805dd28df67d9a9d4625bfddc209841de20.png"
     ]
 }
\ No newline at end of file
diff --git a/lib/iris/tests/test_image_json.py b/lib/iris/tests/test_image_json.py
index d04cc4e1bd..48690ebe44 100644
--- a/lib/iris/tests/test_image_json.py
+++ b/lib/iris/tests/test_image_json.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2016 - 2017, Met Office
+# (C) British Crown Copyright 2016 - 2018, Met Office
 #
 # This file is part of Iris.
 #
@@ -56,15 +56,15 @@ def test_resolve(self):
             return unittest.skip("Less than {} anonymous calls to "
                                  "GH API left!".format(amin))
         iuri = ('https://api.github.com/repos/scitools/'
-                'test-iris-imagehash/contents/images')
+                'test-iris-imagehash/contents/images/v4')
         r = requests.get(iuri, headers=headers)
         if r.status_code != 200:
             raise ValueError('Github API get failed: {}'.format(iuri,
                                                                 r.text))
         rj = r.json()
-        prefix = 'https://scitools.github.io/test-iris-imagehash/images/'
+        base = 'https://scitools.github.io/test-iris-imagehash/images/v4'
 
-        known_image_uris = set([prefix + rji['name'] for rji in rj])
+        known_image_uris = set([os.path.join(base, rji['name']) for rji in rj])
 
         repo_fname = os.path.join(os.path.dirname(__file__), 'results',
                                   'imagerepo.json')
@@ -72,11 +72,10 @@ def test_resolve(self):
             repo = json.load(codecs.getreader('utf-8')(fi))
         uris = set(itertools.chain.from_iterable(six.itervalues(repo)))
 
-        amsg = ('Images are referenced in imagerepo.json but not published'
-                ' in https://scitools.github.io/test-iris-imagehash/'
-                'images:\n{}')
+        amsg = ('Images are referenced in imagerepo.json but not published '
+                'in {}:\n{}')
         diffs = list(uris.difference(known_image_uris))
-        amsg = amsg.format('\n'.join(diffs))
+        amsg = amsg.format(base, '\n'.join(diffs))
 
         self.assertTrue(uris.issubset(known_image_uris), msg=amsg)
 
diff --git a/requirements/test.txt b/requirements/test.txt
index 26a00bcacc..38d7410a9b 100644
--- a/requirements/test.txt
+++ b/requirements/test.txt
@@ -5,5 +5,5 @@ mock
 nose
 pep8
 filelock
-imagehash
+imagehash>=4.0
 requests