Not too long ago I wrote a blog post for Webbeat.tv entitled Website of the day: Tineye which describes the service that allows you to provide an image or image url and tells you where else on the web the image is being used. Tineye will even detect altered images.
It appears as though Google has recently released a similar tool via it's Google Image search. If you navigate to http://images.google.com/ you'll now see a camera icon in the search field, mouse over the image and the tooltip says "Search By Image". Clicking the icon provides a new search field with the options to upload your own image or provide an image url.
I'm a fan of the Tineye service though one has to believe that Google will quickly out perform Tineye with it's capability to crawl websites and gather data more quickly. In either case this is a very useful tool which I am apt to use frequently.