Having done some research in this field- there's a few really useful tools for doing what you want, often known as 'reverse image' search.
www.tineye.com
Even has ability to find source image within a remixed or photoshopped image. Also useful for finding higher res copies of web/thumbnails you have.
(if the image was a composite of a few parts.. then good idea to upload a cropping of your main interested area)
www.picscout.com
Similar to Tineye- but I believe more intended to be hired by the various photostock agencies or photographers to make sure people aren't infringing on their images = might be perfect for finding the match in a given photo agency.
Those are the two big ones I know of- google of course has the giant database behind them, however it's from EVERYWHERE.. and often still using text tags rather than just the image... nevertheless great for finding 'similar' images by formal qualities.