Copyright theft is something I am well familiar with as well. In the past, I had images stolen off my site and reproduced in various items for sale on a cups/mugs type site!

A friend made me aware, and I contacted the owner of the site's legal dept. and they were pulled immediately, and the thief's account closed. I also contacted Google and made them aware of this person's theft.
I've tried watermarks, but as I have found out, if someone is good with an image editing software, they can be removed.
Someone on another forum gave me a tip back then to send digital photos of my work to myself at it's completion. Set up a "free" account such as AOL or Gmail where the space is unlimited. Then you don't clog your reg. email. That instantly produces a record of your work, and no one can argue the date of creation.
With the internet, even though International Copyright laws are intense and in effect, many do not abide by them if they see a way to make a buck off of another's hard work! Grrr.....