Just before anybody spends money, though, I'll toss in what little I know about digital watermarks.
As I understand it, the most common approach is to take your identifying information (a code number), split it into pieces, and scatter it around the image, hiding it in noise. I won't bother explaining the math, but you can think of it as taking specific pixels and changing the color slightly. For example, you can play "odds or evens" with the RGB value.
As the site's FAQ explains, it's nice and fast.
However...if someone knows you watermark your images? Well, just about any change to the image (color or size) changes the RGB values of the pixels, so it's pretty easy to foil. Brighten the image and the red, green, and blue all increase. Crop just a tiny bit, and the software can't find the right pixels anymore.
That's not to say it's a bad idea for some people, but for most uses, I'd be wary of spending money to protect something that you've released as free.
And as a paranoid sidenote, I notice that they don't explain their algorithm. It scares me when security companies don't, because it usually means it's not a very mature system. It's like a shopping mall refusing to tell you how many security guards are on duty; all things being equal, it usually means they're embarassed by the answer.
