My problem was that I had an animated gif that didn't loop. However, the image would only play the first time the page loaded and did not play each time the page was re-visited.
I read about someone appending random numbers to an image in PHP, but PHP wasn't an option on my page. So, what I did was put an ID field on this image:
id="<%# Guid.NewGuid() %>"
Hope this helps someone.