ImageMagick is a nice suite of tools for creation, modification and displaying of images. It can compose, display, and convert images. I recently came to know about its ability to compare two images. I tried it on the images below. I used potato-guy kde version called ktuberling to create the first image,I slightly moved the mouth to get the second image.
I compared them with imagemagick and the result is the third picture. Amazing indeed!
Here is the command I used
$ compare pic1.jpg pic2.jpg result.jpg
For more info look at man ImageMagick