We should have automatic tests for image animation being reset, which should happen when .src. is changed. See bug 594771 for more details, in particular bug 594771 comment 105.
Scott, I feel like you wrote these as part of your work on bug 666446. Is that true?
(In reply to Joe Drew (:JOEDREW!) from comment #1)
> Scott, I feel like you wrote these as part of your work on bug 666446. Is
> that true?
Yes, I did. It's part of the package of tests in image/test/mochitest - specifically test_changeOfSource.html and test_changeOfSource2.html.