Here's a program I've recently stumbled upon that adds an invisible watermark to your picture which is coded into the image itself, so even if someone does "print screen" and copies/crops into photoshop the program can still detect the hidden mark.

I've tested it out and its quite useful. The basic version is free and adds a visible mark bottom of picture (it also adds your unique hidden), however if you sign up they also offer image spider which will search the internet for images with your unique watermark code, if it finds one it alerts you.

[link]

Also there are several tutorials on deviant art that discuss how to add your own invisible watermarks, however they're just layer coded so it doesn't protect against "print screen" copy.

However you can also do visible watermarks much like the one DA offers. It can be editied out true, but most people don't want to spend the time to edit out and then repair/touch up an image, and those that do lack a soul and need to invest their time into learning a skill to better mankind.

Overall, piracy of anything be it music, software, movies, images, there's means to slow it down, but stopping it, is nearly impossible.
You can take steps to prevent 90% of art stealing whores, but for those 10% that truly want to do it, will find a why. Make something idiot proof, you'll just make a smarter idiot.

It really comes down to the community of artists to watch out for each other, if you see someone trying to sell art that you know is not theirs, then take the 2 minutes to e-mail the original artist, or make it known to someone.

So go check out [link]