How to convert Apple iPhone's .HEIC images to .jpg via Linux Terminal.
These commands can be used in Ubuntu versions >= 18.04.
Open a terminal and paste this command:
sudo apt install libheif-examples
Open the folder containing the .HEIC images in the terminal (e.g. navigate there by using cd and ls). Then use the following command:
for file in *.HEIC; do heif-convert $file ${file/%.HEIC/.jpg}; done
Sources: