Yep, the only way you could make it hard to steal would be to deface the scans with some ugly watermark like GM scans have done. It defeats the purpose of sharing them IMO.
Actually, that's not even enough, just for the record. Serj is being nice by disallowing those books.
The only way you can get copyright on something is by investing artistic effort. No, just "messing around" with Photoshop doesn't count. You need to, essentially, create a new work based on the original. For example, if you recolored the images or "updated" the dialogue, that would probably qualify, but might also be borderline, if it was too close to the original.
Just slapping a logo on top? That might give leverage for a trademark suit, since it's arguable that the logo represents you. But that can also be easily chopped off, and it looks ugly.
Think in terms of text, where this is confusing: Under what circumstances do you think you'd win a court case if someone "steals" your reprint of "Hamlet"? Basically, in any case where your version differs in substantial ways from the original text.