Quotes from the TinEye image searching website.
"Why use TinEye?
There are many uses for TinEye, but here are a few:
Find out where an image came from, or get more information about it
Research or track the appearance of an image online
Find higher resolution versions of an image
Locate web pages that make use of an image you have created
Discover modified or edited versions of an image. "
"Can TinEye find similar images? Does TinEye do facial recognition?
TinEye finds exact and altered copies of the images that you submit, including those that have been cropped, colour adjusted, resized, heavily edited or slightly rotated. TinEye does not commonly return similar matches, and it cannot recognize the contents of any image. This means that TinEye cannot find different images with the same people or things in them. "
"Why can't TinEye find my image? I know it is on the web.
There is a huge and growing amount of images on the web, and TinEye has only crawled a fraction of them so far. We probably missed your image because we have not crawled the website(s) where it appears, or we don't have the latest updates from the site(s). Additionally, some website content is protected by robots.txt or is otherwise uncrawlable.
However our search index does increase incrementally over time, so please be sure to search for your image again at a later date. To receive notification when we update our index or to get the latest news, please check out our What's New page, which also offers an RSS feed. "