I think the only clear way to prevent people from making tubes of your art work, is to not post your artwork up on the very "public" Internet. Unfortunately, anything you post is pretty much fair game. I agree with Imckenzie that maybe trying to get people to at least give you credit or giving them a form to fill out for permission to use is a start. The thing about tubes, then, is that pretty much all of them are "illegal", because unless the person makes a tube of a picture they drew or created, or the tube is from a copyright free image, they are all from an image of something, from somewhere. Me, personally, as long as the person isn't selling something and making money off of it, I don't care if they use what I post on the Internet, because I assume once I post it publically, it's "out there" and people are going to use it if they like it. Of course, legal issues and copyright issues are another can of worms altogether. Before the advent of MP3s and digital music, what did the music companies do about people tape recording music off the radio?