At my $OLDJOB, I used ImageMagick to compare snapshots during some automated front-end testing on our public Drupal site. It would compare the running test to previously accepted images, highlighting pixel diffs in red. It would also generate a 4 frame animated gif with of the original, highlit changes, the running test version, and back to the highlit changes (so it could loop for better comparison).

Imagemagick saved enormous amounts of time for us as we made CSS and other module upgrades.