Aside from the usability concerns for people with differing sensory delivery needs, the perl modules in the ImageMagick package may be able to perform such effects on things in an efficient way. The specific perl tools are located here.

All of the discussion on audio format implementations of CAPTCHA are interesting but not so nice on a braille reader. To this I propose live chat operator conversation as a potential alternative, but I'm sure some method of exploiting this using IRC/IM bot techniques may confuse anyone set in as a further alternative to verify the humanity of the querent.