If you are like me, you wonder who and how your photos are being used online. The best way to find out is to use TinEye which is a reverse image search engine. You can submit an image to TinEye for free to find out where it came from, how it is being used, if modified versions of the image exist, or to find higher resolution versions. TinEye is the first image search engine on the web to use image identification technology rather than keywords, metadata or watermarks.
I use it periodically.