Well, if you want to track theft of images, a digital watermark is the way to go. The MarcSpider service searches for digital watermarks, and logs them. The digital marks will survive most image conversions (bmp to jpeg for example) and at certain settings will even survive being printed out and rescanned. An optical one would only be useful for "demo" images, as once you sell someone an image, you certainly can't have an optical watermark on it. But a digital one won't interfere with the image itself.