Tin Eye is an image-based search engine. The way it works is you submit an image (via upload or by entering a URL) and TinEye returns other versions of that same image that it finds on the web. I’ve used successfully this in 3 ways.
1) I’ve used it to find higher resolution versions of an image I only have a thumbnail of. This is especially helpful when you’re making a poster or a print of some sort and you need something with a higher image quality.
2) I’ve used it to find full versions of images when all I have is a cropped version. For example, I want a picture of a $100 bill, but all I have is a crop of George Washington’s face.
3) I have an single image that I know is from a set of similar photos. If TinEye can find a page with the image I already have, often it will be a page that also shows the other photos from the same set.
Give it a try. It’s still in beta, and it does require you to register for free before you can use it, but it currently indexes over a billion (that’s Billion with a ‘B’) images. So check it out at http://www.tineye.com