Some time ago Google Images started to let you find similar images than one selected, just by dragging an image to the textbox (try it: saerch an image, then from the results start dragging an image toward the textbox on top of the page, and another box appear... you simply drop the image in it, and G searches for similar images)
But I just notice that you can search for similar images as an image you have on your desktop! Open Google Images, and drag an image from your desktop (or elsewhere on your harddrive) and drop it in the searchbox. Voilá!
this works in chrome + mac os x 10.5... but I shall try other combinations.
Another great thing is that the image does not have to be called something special, G guesses what your criteria should be to have similar answers.