Digital watermarks offer some degree of identify ownership but won't help you search for them. People really believe that everything on the Web is in the public domain. It's amazing to find who rips stuff off. I encourage them to go ahead and use the art, but give me a credit, and link back to the Design Center. In many cases, I'm happy to report, they do just that.
Court is a drastic measure.
First, you'll be called upon to prove that you've lost income due the infringement. The costs can be staggering. Sometimes in instances of commercial infringement, you want to protect your property because of brand name, and lost sales. In these cases you can usually justify the cost of an attorney. If you're a professional writer or artist and your work is being used for content in another site, you've probably got a good case.