Difficult question. It really depends on what your customer base can
deal with. If they are blind, there isn't much I can do for you. ;-)
http://code.google.com/p/kaptcha/wiki/ConfigParameters
That url outlines all of the various things you can tweak right now.
To start off with, you may want to change the font type, style, color
and/or change the characters which are used.
I also suggest that you implement the ability to click the image to
get a new one that I describe at the bottom of the howtouse page.
http://code.google.com/p/kaptcha/wiki/HowToUse
jon