I'm not sure what you mean by having an admin apply it to the messages. It would be a profile option. You would not be allowed to modify your post to look like it was written by someone else, if that's what you're asking for.
Having a special image for each forum could be cool too. I had briefly thought of that too, I just wasn't sure if I wanted to go that far. If I were to add that, it would probably just be a text field to enter an image URL, just to keep it simple. (I'm still working on the proper ways to pull in profile images based on the way SMF handles them.)
I was thinking that the system should be built to support any number of categories, but probably would only be allowed in areas like Forum-based Gaming.