it would definitely be difficult. If the pictures weren't ever recycled then image rec could probably start to make dents (but in general, it'd take some serious recognition. They could go to the site and learn all of it's pictures - but cycling pics in and out regularly would fix that). Making you pick several makes it even more clever. The less things that remain constant the harder it is to defeat. And it doesn't ahve to be perfect, just good enough to make it unprofitable for these folks.

The only thing I suspect is that there's a set number of solutions for any one. A computer could bang on it passing in random stuff. It would take a while to get through but it's doable. However if you switched it so that the number of right answers changed, the number of total pictures changed , just really worked at making each new page load really distinct, this should be able to really put a hurtin on the spammers without killing humans in the process.

I think the original idea for this came from a project called kitten auth:

http://www.thepcspy.com/kittenauth

http://www.thepcspy.com/articles/security/the_cutest_humantest_kittenauth

http://www.thepcspy.com/articles/security/improving_kittenauth

Problem is, what if you're blind?

Is the same fix for blind as it is with CAPCHA I think (play to read a message that will explain what to do like what #'s to press...not sure how I would implement it).