Check out Retrievr (via Matthew Kirschenbaum). It allows you to find images in Flickr's archives by drawing in a little box. You can play with black and white (and find B/W photos), or add in color to search color photos.
I tried doing my best version of George W. Bush:
I didn't get the President, but I did get this endearing floppy-eared creature.
Then I tried a heart:
And I got this result, which is pretty good.
According to the "About" page, the plugin isn't designed to recognize faces or objects, just shapes. So the result is equally good (or bad) if you use obviously abstract shapes:
That leads you to this. Not bad, eh.