Thanks to a friend of mine (Daniel Bridge) telling me how, I recently discovered that one of my water spider photos was being used on a blog despite it being copyrighted and a couple of my fox photos on a Creationist Wiki!
I mentioned this on Twitter and some folk asked how I did it, so here is how:
1. Open a new window on the Google images page: http://images.google.co.uk/
2. Open up a second window with the photo you wish to search for. Then drag this image to the search box in the second window.
3. Then Google will show you a list where your image and similar images have been used on the internet.