We sometimes need to find the source of a particular image . some guy post some ultra cool photo and we wonder hey where the hell did he get that. no need to ask keep your dignity intact and use Tineye search engine..It shows which web pages contain that image and also similar images. say if there is series of photos tineye shows them all!
In addition if you are firefox user you can easily add the Tineye Addon. so that you can right click on an image an search for the source by clicking context menu item in firefox.