I have found most of the information referred to in my origial question. However, one mystery remains: Who is userID -1? Almost all of the offending files are created by userID=-1. Help here is appreciated.

Thanks Sebastian. THat helps somewhat. Now the question is how can such a user cause image files to be saved in my file space? I played around a bit and see that one can download a 'photo' when registering. However, if the registration is not accepted, nothing happens to the photo. It stays in the User space. Looks like some kind of a bug. However, now come the question, how do i get rid of such an image? It is indexed in several tables, the Files table among them. I can of course, delete them directly from the database, but there should be a more correct way.
Hans