I'm going to release a Mosquito 0.2 in several days with a really cool feature. It will be able to save some screenshots as as a "reference" ones. Later, while crawling your site again along with new screenshots you'll get difference heatmaps generated which will make it able to spot diffs in a matter of seconds.
Here're two samples: Opera Signup Page Diff (reference image, current image) - you can see a layout bug in the footer was fixed. And 3 screenshots from the site I'm developing in my spare time: diff, reference and current images. You can spot even a functional change here.
So, stay tuned. Mosquito 2.0 is about to be released in a couple of days!