[olegk]

My spam script would always answer "cat", so among 8 options (cat, dog, house, drum, road, tree, book, horse), I'd get a 12.5% success rate.

Plus you constantly have to update your image library, which a huge pain.

Also, recognizing 10,000 images will take me around one day and less than $1000 with Amazon turk, thus giving me a perfect 100% success rate. After that you would have to completely renew your image database.

[mseebach]

sigh .. etc. There would be more than 8 options, many more.

YES, it would be a pain to update the library, which is why I'm commending this particular concept for solving that problem ...

[StavrosK]

You aren't getting it. The probability is 1/<number of options you present to the user>. If you show the user 100 images and ask them to select one, a bot will have a 1% probability to find the right one, but the user will tell you to get lost.

If you present 10 images (still a stretch), bots will have 10% success rate just answering randomly.

EDIT: Wait, from what I see you mean that the user will have to write "cat" or "dog" or whatever? That's better, yes. Communication, however, is hard, which is why me the GP didn't understand what you meant.

[Ilovepee]

not to mention the fact that the bot will get spotted for entering the same phrase more then a few times, get put on a list and get served the squiggly crap

[olegk]

You have no idea how spam works. A bot isn't just one user trying to enter "cat" repeatedly. Botnets send requests from thousands of different IPs. You wouldn't know which ones are real users, and which ones are bots.