If you take a lot of pictures and put them up on the web, sooner or later someone will pinch one in order to use it themselves. They may mean to steal it, they may not, but either way, it's out there. 'Who stole my pictures?' is a Firefox addon that you can use to try and identify use of images elsewhere. Simply find your image (say on your photographic file sharing site), right mouse click and choose the 'Who stole my pictures?' option. The resource will then search through 5 different image search engine sites trying to find it. What you do then is up to you of course.
Now, while it's useful for that, and of course that's its primary use, it's also great to see who else IS using your images - I tried it out with some of my Library propaganda posters and it was fascinating to see where they were being used. Of course, I'd given permission for use so I wasn't trying to hunt anyone down! What I also found it was useful for was finding similar images as well, and so with a bit of lateral thinking this could be a great little search tool.
The only slight downside is that it seems to be Russian, but there are no instructions on use, it's all obvious. If you're into images and use Ffox, give this a go.