My student Arjun Satish pointed me to a new image search engine on the Web — www.tineye.com — which takes an image or a url from you and searches for that image on the web. It claims to look at 701Million images. I played with it.
It seems to use some metadata because it does find if the image does exist on the Web but if you give it one of your images it correctly tells that it does not exist on the Web. I t tells directly that it is not doing any content based search.
It is good in what it does.