This topic has come up before, but today was the first time I have tried to find out if some of my images are being used on other websites. I heard of a site called Tin Eye today while listening to a podcast. The site is still in beta, but you can request an invitation to test out the site. I signed up, and within the first few minutes of using the site, I found one of my images on another website from the UK.
The image I found was one that has sold over 500 downloads on iStockphoto so I wasn't real surprised. I was surprised at how easy and fast the site was to use.
If it sounds like something you might be interested in, give them a try.
www.tineye.com
Edit:
I played around with this a little more now that I'm home. I know it's still in beta, but I would think that it should find images that I have posted in this forum, but it didn't.