If an image is interrupted while downloading, the part of the image already downloaded is cached and used as the "complete" image for future visits to the image. Screenshot (http://www.dzm.com/pics/broken_image.jpg) was created by allowing image to partially load, then moving mouse over different mouseover, then back.
This is a case of bug 30852 - images are not refreshed on a reload. Here the incomplete image is getting put in the cache and never refreshed. *** This bug has been marked as a duplicate of 30852 ***