I'm not at all implying there is anything at all wrong with the method Brian suggested ... this is just to give you an option since one of the greatest things about AE is that there's several ways to do everything ... it's good to know multiple methods since each has pros and cons that can make it more or less suitable for different needs.
You could also precompose the image layer and leave its attributes in the main comp (so the animated keyframes don't move into the precomp, which is a choice you're given whenever you precompose). Then, you can change the image in the precomp as many times as you like. Some people consider a downside of this method to be that you need two comp viewers to see what you're doing, but I always work with two viewers anyway so it's a non-issue for me. The upside of this method is that, if you have to change the image a hundred times, you don't have to wade through all hundred layers in your main comp.