Have you checked out Google Similar Images yet? It is pretty cool, and fun to play around with.
Google Similar Images “allows you to search for images using pictures rather than words.” Basically, you search for an image, say nuts. You will get some cool results, like a picture of large seeds, a squirrel with large testicles, and a horse with even larger testicles.
Under the images, you have a link, Similar Images. You click that link to show images that are visually similar to the image above it.