Google Image Search added a feature launched by Bing in March: showing more sizes for each result. If you mouse over an image, there's a new option called "more sizes" that lets you find out other versions of the same image. This is useful if you want to find the higher-quality version of an image or if you want to list all the sites that include a certain image.
Most likely, Google uses the same technology that lets you to find similar images, but restricts the results to images that are almost identical if you ignore the resolution or some minor editing.
A similar service is TinEye, a search engine that finds the different versions of an image you upload. "TinEye is a reverse image search engine. It finds out where an image came from, how it is being used, if modified versions of the image exist, or if there is a higher resolution version."