http://poseidon.csd.auth.gr/signatures/
The distribution of graphic images over the Web raises serious questions about copyright protection in this digital realm. This site offers an approach to providing both copyright protection and ownership recognition. A set of program files can be downloaded for various platforms. Once set up the program casts an id_number as an invisible signature on any digital image. At another time the owner can examine whether the image which is suspected of being copied has it's own signature id_number. Currently the platforms supported are Silicon Graphics, Sun Microsystems, Hewlett Packard, and MD-DOS.