What is to stop a bot from just clicking randomly? A 1/6 chance of registering an account is good enough for botting purposes. That's the problem with these multiple-choice solutions. They simply aren't strong enough protection against a flood of tries.

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.

Flickr is harder, I'd use a different database.Doing image searches using existing online images is becoming easier (e.g. TinEye). An evil bot would just do image searches against flickr for each of those images and read/scan the tags there to be sure.

Is that really effective? Just take some face detection code and have it learn what cat fur looks like instead.

It's ok effective judging by my usages and I could just iterate over this solution if someone brute forced it or implemented a "cat fur detector" (which for my uses is pretty unrealistic).

Battling spammers is really a battle where you implement better protections and they implement better attacks. The conclusion so far has been that it isn't possible to check-mate them - - and it's very unlikely it will ever happen as a lot of spammers use "human bots"...

Using visual captcha is much more user friendly thought, so it's a win for the users.

There is actually a 'Google Tech Talk' on detecting cats (versus other shapes) from images:

http://www.youtube.com/watch?v=-w72_VwSj6A

  Is it really that easy?  I would think flagging for texture would be difficult, as it's
 likely indistinguishable from noise (data, shadows, clothing..).  Approximating shape/avg.
 contour is likely easier than looking for fur patterns.

  Any suggestions on books/papers/etc on image recognition?