Google already offers search by image feature and better late than never, Bing now offers the same functionality. Microsoft’s search engine will now allow you to search for a particular picture by using an Image Match button. When you click on this button, Bing will show you a list of different size images that you can then narrow down by size. On the other hand, Google’s Search by Image works by dragging and dropping a photo into the text box. It’s certainly an useful feature, when you are searching for a picture in a particular resolution.
SOURCE: Bing Search Blog