nice, but the problem I see is this: how would you create the images?
in the OP you say "The p2p system will produce CAPTCHA images...". Then on another you said "Your computer will not produce the image for you"
so... how would the image be generated?
the problem is that the computer, person, or whatever, that generates the image already knows the answer, so it could "cheat" and create a new block and earn $ without effort.
and if the image is created in a way that nobody knows the answer, then... how could the system verify that the answer is right?

sorry, a captcha cryptocurrency is not possible unless you figure this out