Unfortunately, it isn’t easy to protect your photos once you’ve uploaded them to the web. Your ability to control how people use your images depends on a few things.
If you’re posting them on a photo-sharing site like Flickr or Shutterfly, check the terms and conditions of the site before you post; you may be giving those sites broad license to use your photos. The T&Cs may also describe what steps the site will take in preventing people from copying your images.
If you’re posting them on your own site, you still have to worry about people downloading and copying your photo.
You don’t have to register your photos to receive copyright protection; as soon as you create the photo, you’ve established copyright in it and can use the law to prevent others from using it. However, tracking down the person that has copied your image can be difficult, and sending them a cease-and-desist letter or even filing a lawsuit may be prohibitively expensive depending on what has been copied.
Another approach is to reduce the quality of your photos; by dropping the resolution of your image, the incentive to steal your photo might be lessened. Big photographs or smaller ones with high pixels counts are more likely to be copied. Of course, your purpose for posting the image may be to show it off, so this might not always be the best solution.
You can also place a © symbol with your information by the photo or place a digital watermark in the image. This, together with a lower-quality image, may be your best bet in preventing copyright infringement of your photos. Once the image has been purloined, though, it can be difficult to control its movement from there without legal action.