Several people, including some other AT members and myself have had trouble with IE6 loading GIFs after installing Windows XP Service Pack 1. The images will appear if you manually right-click and choose "Show Picture".
After doing some research it seems that it's actually IE6 SP1 that has the problem, when you install XP SP1 it automatically updates IE6 to SP1.
There's no known cause for it that I can find and it only affects a few people, the only fix seems to be using plain IE6 without SP1, assuming of course you are having the same problem.