convert

# To convert a file from jpg to pdf
convert original.jpg converted.pdf

# To resize an image to a fixed width and proportional height:
convert original.jpg -resize 100x converted.jpg

# To resize an image to a fixed height and proportional width:
convert original.jpg -resize x100 converted.jpg

# To resize an image to a fixed width and height:
convert original.jpg -resize 100x100 converted.jpg

# To resize an image and simultaneously change its file type:
convert original.jpg -resize 100x converted.png

# To resize all of the images within a directory:
# To implement a for loop:
for file in `ls original/image/path/`;
    do new_path=${file%.*};
    new_file=`basename $new_path`;
    convert $file -resize 150 converted/image/path/$new_file.png;
done