I am vision impaired & *had* a hard time with captchas.
Until I remembered that in firefox, ctrl+ zooms.
When I run into a captcha, I hit ctl+ a few times, fill out the captcha & submit.
Then I hit ctrl- a few times to get it back to the appropriate size. Yes, I know about ctrl0, but I already run most pages a little zoomed.
Every single person I have shown this to, vision impaired or not, no longer has a problem with captchas...
pass it on!