[unnouinceput]

I see that as an absolute win. If you're having problems understanding simple math questions then I won't want you as my user in the first place. Morons out.

As for visual impaired ones, I agree this one is harder to crack. Usually you do it by audio, which in itself is more then 50 lines of code, but here is my personal approach. Absolutely none is stopping you to have, for visual impaired ones, a separate step like the one described in OP, where you have mail activated. You see visual impaired users have infinitely more patience then normal "visual" ones. They are used for web to not be friendly, so they won't mind going through extra hoops if they want your service. So a checkbox saying "I am visual impaired and I want registration by e-mail" or something equivalent and you're good to go.