If you don't have a comment that can help solve the problem then don't post. A lot of people use IE7, there are even people still using IE6 so he needs to solve this problem.
I'm not sure how you can fix this but there is an easy work-around and it would have a better outcome than using javascript and gif files
The work-around is that you animate that in Flash and use AS to tell the animation to go backwards or forward (It maybe more complicated than I put it but its actually not that difficult :S)