There's just one problem, Arek. Do you really think people are going to want to dig through the website to find the answer, or even dig through 150 different images to find the one that looks like what was pictured? Considering the rash of seemingly ADD-riddled people that've been getting more and more common, I sincerely doubt it.
I like the idea of the user having to fill something in. But, what if the question at the end pertained to some aspect of the sign up process (something like "What word follows, 'By clicking 'submit', you agree to these _______'?", or maybe a fill-in-the-blank setup along the same lines)? Then the answer is either right there on the screen, or just a "Back" button click away. Then you still have the user needing to look for an answer (thus making it a greater chance that it really is a person and not a bot), but it makes for less of a wild goose chase through images, tunes and forums.
There could be a link to the project under the pic so that way they can find the correct answer.
JUST AN IDEA MAN STOP HATIN