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 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.