It has a choice of using three opensource/free engines - SecurImage, ReCAPTCHA or (my personal favorite) MathGuard.
I know you are thinking about integrating new Captcha into a later JoomlaComment version Daniel, perhaps it would be easier to integrate this extension?
I was looking at the code myself to see if I could do it, but it's a liitle bit advanced for me What do you think?
EDIT: The SecurImage is pretty decent for a simple Captcha engine as a new default, and is Lesser GNU v2.1 licence.
is the gallery for that engine, the one at
is actually very good according to my own knowledge of spambots - it uses GDF fonts so the "dots" in the font are random, it really messes with Spambot OCR.
I'm very interested in the Mathguard myself, is it much better at blocking spambots than the current basic captcha code? I noticed its not on the devsite Issue Tracker yet so I take it it hasn't been assigned.