All original image work is automatically copyrighted by its creator. However, LEGALLY there is no real recourse other than a 'cease and desist' without a visible opyright symbol. And better yet, registered opyrights hold infinitely more sway in court. Registering a opyright can be done in the U.S. for $30 via the U.S. Copyright Office (see http://loc.gov/copyright).

If your image has been stolen for profit, AND YOU CAN PROVE IT, you are far more likely to receive compensation with a visible registered opyright.

Caveat/Warning:
U.S. Copyright Laws do not apply everywhere, so goodluck if your goods are stolen by individuals from most Asian continent nations (China, Russia, etc). Its one of the drawbacks to the internet.

Lastly:
This is a digital age. ANY image can be modified to remove Watermarks, Trademarks, Benchmarks, Stretchmarks or Skidmarks with time and expertise. Just because a opyright symbol can be cropped/blurred/erased/etc. does NOT negate it. It merely requires you to provide proof of orginality. Posting here at Renderosity timestamps your image. Conversely, it opens you up to being ripped off. Your choice.