Just a couple of minutes ago I've updated my profile on Amazon by providing an alternate email address. The main problem encountered during this procedure was recognizing the correct character sequence in the CAPTCHA image. I had to try several times before typing the correct sequence. So my point is: if a sighted user has encountered so much problems with CAPTCHA images, what will happen to a visually impaired person who tries to fill in the form?
I'm not talking about blind users who can still get an alternate version of the CAPTCHA. I'm simply talking about persons who do not have a perfect sight. Honestly, I was frustrated by this. I kept typing my password, my new email and then.. nothing! Or, even worse, an error message claiming that the characters inserted weren't correct. Now you may ask if there's a feasible alternative to images. The answer is... yes! Simply put, use a textual alternative, for example by asking a simple question that only a human reader can understand (e.g. "What is the capital of the United States?").
What's more, you can actually get rid of a lot of server-side procedures used to creating, changing and inserting the CAPTCHA image. The only problem with this approach lies in the correct choice of some good questions. Anyway, surely we'll prevent our users from being frustrated when they try to decipher the CAPTCHA image and get some error.