Wow, that looks interesting.
They're very reserved with info about it - on their site they just say "Various types of image files can be compared
" - you can see from that link they tell you how much RGB there is in each image, so even though in the screenshot they look the same, they arent.
I may as well show it here sure ;-)
can it compare PDF files too - in a viewer I mean ?
Kartal mentions graphic files - I presume the viewer only does images (not PSD, not vector file formats)
 corrected Kartal's name