hmm, are you saying that you want a script to intergrate into your bulletin board
so that that only the registered user can upload a image??
assuming that, it would be better if you use phpBB bulletin board, which has the feature included, other then this if you're using your own custom made bulletin board
you will need to use sessions / cookies plus abit of programming to code this, making a upload form is not that tough so the only part which will consume time is coding a secure upload backend.