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.

When you submit an image to be searched, TinEye creates a unique and compact digital signature or ‘fingerprint’ for it, then compares this fingerprint to every other image in their index to retrieve matches. TinEye can even find a partial fingerprint match.

TinEye does not typically find similar images; it finds exact matches including those that have been cropped, edited or resized. I have found TinEye very useful, you can easily find out who has stolen your images from your website or portfolio. You can see how powerful TinEye is by checking out their search examples.

Source: http://tineye.com/