One issue that you seem to be forgetting with this problem is that it could prevent some legitimate humans from accessing sites as well. Reading an image breaks the expected flow of information, and would cause screen readers and other such things to fail. I think that paypal offers a sound clip of someone saying a word as an alternative for blind people, but I recall some sites that don't offer this, and there's still the question of whether sites can be made sufficiently accessible while excluding the automatons.