What is the GIF exactly? Depending on what it is, you might be able to get away with either scripting or CSS animation.
That’s what I was wondering. It could be that you don’t need a gif at all. Now, if it’s a gif of cat video, that’s going to be a problem.
You can use scroll magic or waypoints to control just about anything with scroll. However, a gif is another animal altogether.
One thing I would add to @shaneisme’s idea is to have a container that houses a static version of the gif, obviously the FIRST frame. Then, request the gif at that scroll position, using a mixture of JS and a plugin like waypoints. Then, with a little bit of server-side magic, it can be downloaded and inserted with a HIGHER z-index than the static version… or just in place of it. However, to respect people with slower connections, it’s probably better to do the former, so they have SOMETHING to look at.
