I'm afraid I just don't get what the big deal is. Sites are ripping off the HOL. It sounds like they are trying to make money off of work that isn't theirs and are hardly ethical about it. To do nothing lines the pockets of people ripping off others' work.
The only solution to stop this seems watermarking. And not invisible marking -- that would only help if one were trying to sue a web site or something of that nature. A visitor to one of these unethical sites isn't going to load up images in Photoshop just to see if there's a watermark. But a visitor that sees 'Hall of Light' on all those pics is at least going to unmistakably know their origin. A layer of registration would probably just have the sites rewrite their scripts.
Some of the other issues I've seen raised don't really make a lot of sense either. Someone that took the screenshots for HOL took them for HOL, what HOL does with them is their own business. And if you took those screenshots, you'd already have them sans watermark. If you didn't take those screenshots, I don't see how you can complain about how you can use them.
And, to top it off, if someone wanted to make an icon pack or something like that, they've said they'll provide them without the watermark.
I do agree that it kind of sucks as a casual visitor to HOL to see the screenshots 'marred' but it doesn't really take away from its functionality.