If he's doing the work for someone this what does he expect? He's selling the images to them right?
You can easily get around that with viewing the source and finding the link if it's HTML or a simple screen capture if it's a flash. If it's really an issue then just give out a small resolution sample with a watermark on it.
Bingo. Even though you can modify it but if someone gets caught doing that they are screwed...
This guy was awarded $1.3 Million for using his photos beyond the contract:
http://www.pdnonline.com/pdn/newswir..._id=1003650053