Let’s say you have a folder named before which contains original images, and a folder after which should contain visually identical images with the same file name. Using ImageMagick’s compare, you can do this:

for file in before/*.png; do
    result=$(compare -metric AE "${file}" "${file/before/after}" /tmp/diff.png 2>&1);
    if [ "${result}" != '0' ]; then
        echo "${result} incorrect pixels in ${file}";
    fi;
done;

ImageMagick compare -metric AE exit status is non-zero if at least one pixel color differs.

Let’s say you have a folder named before which contains original images, and a folder after which should contain visually identical images with the same file name. Using ImageMagick’s compare, you can do this:

for file in before/*.png; do
    result=$(compare -metric AE "${file}" "${file/before/after}" /tmp/diff.png 2>&1);
    if [ "${result}" != '0' ]; then
        echo "${result} incorrect pixels in ${file}";
    fi;
done;