With a new image search engine from TinEye you can now find out. No typing in keywords, descriptions or tags, TinEye will first take a look at your image and then search the internet for a match.
The folks at TinEye claim that even if your image has been cropped, resized, or "photoshopped" they can find it.
You can click on the search results and it will toggle between your mage and the image found to compare them, a pretty cool feature.
To make life even easier, there is a plug-in for Internet Explorer and Firefox that allows you to simply right click on an image on any webpage and perform a search for that image.
You can view some searches and the results here and here.
