Thanks. I sent him this url. I hope he'll look into it soon.
It appears that 'by default' some data directories are owned by Apache and some by my FTP username (so without having made changes to any of the above yet). Since I don't create the data directories myself (PhotoPost does) I am wondering why some dirs are owned by apache and others not. Does this have to do with the setup of my server? Would it cause problems if nothing is changed? I think this is why I'm having problems removing photos, sometimes when I remove photos from the site then they are not being removed from the server.
Something else: I am using Classifieds as well. I suppose that if I would make PhotoPost more restrictive by adjusting the ownership and permissions, that I'd have to do the same thing for Classifieds?