A software to invisibly sign images

in additon to all what other fellow users suggested, There is a Software that you can use to sign you image with an invisible seal and they also have a service that searches the internet if someone uses the images you protected through their service, I wrote a full review of this and I would not want to jam this thread so this is the link

http://www.ralphtips.com/2009/08/protect-and-track-your-images-online.html

The software Homepage is http://www.adptools.com/