the real challenge is to collect enough pictures so that your database could not be broken by brute-force. Of course you can use Flickr's tag to identify it, bu

the real challenge is to collect enough pictures so that your database could not be broken by brute-force. Of course you can use Flickr's tag to identify it, but there are many mis-matches. Google's method solved this problem because image orientation is a heavily studied problem in computer vision which could take advantage of computer to help creating large visual database and then prune out the easy case for computer.