So if I have a library of 10s of thousands of images of animals, shapes, things etc. that are all easily recognisable to English speakers -- e.g. cat, dog, house, drum, road, tree, book, horse -- and ask them to write in what is it, AND I constantly update that library and retire pictures that's been used many time -- what is your dumb script's success rate?
What if I combine three pictures in each challenge - e.g. "cat house triangle"?
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.
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.
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
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.
If you display 10 random animals (or shapes), and ask a user to pick the right one, a dumb spam script would have a 10% success rate.
If you display one image of an animal and give 10 possible answers, a dumb spam script would have a 10% success rate.