There's this website where you can upload the image and they give you links of where the same or similar images are online. It's really very handy to find out where an image came from.
http://www.tineye.com/

Wow, thanks. I see they have a Firefox add-on that looks useful, too.

Yup i just installed that addon a few days ago! =)

I have used that site to help me find when people have stolen my images. I've been able to get most of them to remove the illegal images.