As for diff'ing two images (image A and B), assuming there's a picture that represents the diffs (image D), if you did a pixel by pixel comparison of the two im

As for diff'ing two images (image A and B), assuming there's a picture that represents the diffs (image D), if you did a pixel by pixel comparison of the two images (A & B), if the two pixels differed in color, you could simply display that pixel in a complimentary third color on image D (or allow the designer to see all conflicting pixel in a single, consistent color which would show clusters of colors where changes had been made). Just a thought.