They are associated with different "levels" in the forum system. I used the green stars to represent donators though I will eventually replace them with a custom image.
The five blue stars are for moderators or something. Otherwise, I think the star count goes up with "number of posts" levels or something.
By custom image... do you mean that the user can upload their own little symbols to use as the stars?
No, I mean that I will upload an image to use instead of green stars. There is no way that the users can upload their own icon, unfortunately... but then, you already have profile pics.