Rick Scott: I do agree that the problem is probably very limited. And the fact that the image is the same every time (so far) does make it easier.
However, this is a site that focuses a lot on usability, with which I feel accessibility is pretty tightly connected, so I think it is relevant to consider that the solution, although widely used, isn’t anything near perfect.
It’s been a pet peeve of mine since I had to help a blind friend sign up for at site that used this technique. There has to be a better way of protecting against spam-bot, although I am not myself smart enough to find one.