I have another copyright-related question that I think deserves its own thread. Some companies that sell public domain art that they've scanned warn that their images are protected by "invisible watermarks."

I just wondered how useful this would be for protecting my images. Is it easy to create invisible watermarks? More important, is there a way they can be used to remotely track down pirated images, or would you have to spot a suspect image first, then visually inspect it for an invisible watermark?