TinEye lets you search your online images and see where they're showing up... (whether they have your permission or not!)

"TinEye is a reverse image search engine. You can submit an image to TinEye 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."

TinEye, here.