The jam has no special rules for copyright or IP use, and makes no claim on any entrant's works.

1) You own art you create wholly yourself, and can do whatever you want with it. You retain ownership of your works after the jam.

2) Nobody owns public domain artwork or IP, so you can do whatever you want with it, except license it (though I think you can still license your derived works)

3) If you have a license to use someone else's artwork or IP, then you can do whatever the license allows you to do with it. Be sure to check whether it allows you to modify the art, if that's something you want to do. Be sure to check whether it allows use on multiple projects, if that's something you want to do. I've never personally seen a license that restricts use of artwork to a single project, but I have seen restrictions for a single *commercial* project, where that same license allows infinite reuse on freeware projects - but still check the license to be sure!

4) If you don't have a license to someone else's artwork or IP you cannot use it.