The search box on Google Images now sports a camera icon on the text box as an indicator that you can use to either upload an image from your computer (by just dragging & dropping with browsers supporting HTML5) or specify an url of an image that you want more info on. This image is then used as an search argument and the web is searched.
This kind of visual search is already available with the Google Goggles mobile app. Although the results are not perfect in all cases, this is a remarkable development in terms of bringing complex computer science topics to practical use.