You can compare two text files very easy with
diff and even better with meld:
If you use diff for images, you get an example like this:
$ diff zivi-besch.tif zivildienst.tif Binary files zivi-besch.tif and zivildienst.tif differ
Here is an example:
Original from http://commons.wikimedia.org/wiki/File:Tux.svg
I've added a white background to both images and applied GIMPs "Difference" filter to get this:
It is a very simple method how a diff could work, but I can imagine much better (and more complicated) ones.
Do you know a program which works for images like meld does for texts?
(If a program existed that could give a percentage (0% the same image - 100% the same image) I would also be interested in it, but I am looking for one that gives me visual hints where differences are.)