I came across a neat little website today called TinEye (www.tineye.com). According to their website, TinEye is a reverse image search engine. It finds out where an image came from, how it is being used, if modified versions of the image exist, or if there is a higher resolution version.
One neat thing about TinEye is it finds variations of images. For example a demonstration video on their site shows a search for a picture on the Mona Lisa. Not only do the results come back with any websites that have the Mona Lisa image displayed but also sites that have pictures that look like the Mona Lisa. Here is the demonstration video