Image to verify user before posting

Maybe PunBB does this already, or there's already a module that I'm not aware of - I'm new.

I'm thinking of not requiring registering a user name (quite a hassle for occasional/random posters.) Instead, display an image with letters and/or numbers so that the user can type in to verify that he/she is a human being to avoid spam. Is there a solution for this? If not, this is my feature request