Hi César,
Your approach to generate image captchas to openacs forms is very easy to implement. I just downloaded tclmagick simple build from Praganas' website as you suggested and unzipped it inside tcldir in my linux box.
The only additional step i did was create a validation block for ad_form in the tcl page script:
} -validate {captcha_text
{[captcha::validate $captcha_id $captcha_text]}
"Text must match!"
}
}
It works fine!
My only concern with this approach is inaccessibility of image CAPTCHA to users who are blind, have low vision, or have a learning disability such as dyslexia.
My question is: are you working on some kind of alternative to image captchas like audio CAPTCHAs or other? Maybe it could be a good openacs web accessibility project for google summer of code 2007 (see http://www.openacs.org/xowiki/GSoC).
Alternatives to image captchas see this W3C paper:
http://www.w3.org/TR/turingtest/