http://www.digimarc.com/products/myp...rc/default.asp
A watermarked image can be manipulated somewhat (res<b></b>ized 0.6X to 2X, rotated) and still contain a recoverable watermark. They have a spider that hunts for images with your watermark on them, so you can see any unauthorised use.
Has anyone creating original images intended for exclusive use on one or two sites only ever used this system?