I'd say you'll have an impossible time trying to stop or prosecute someone for taking a few gradient images. I'd try not to worry about it too much - at least he isn't stealing the entire design. You can try sending the webmaster a threatening letter/email - that sometimes works. Just make sure that the letter looks professional/written by a lawyer to make it effective.
Contact him first. If he doesn't respond, contact his host and hold them liable. I had someone who ripped my whole layout and I didn't get an answer from him so I sent an angry letter to his hoster.