41182 CAPTCHA (image verification)

I'm getting spambots using my signup cgi script on [login to view URL] and sending rubbish emails. I have disabled my script and the old version is here at 100for100 dot com / old_signup.php. The cgi script used is free "anymail script" check google. I need someone to add CAPTCHA to my signup form and hopefully block these spambots from using my script. I am on Linux server, running php + mysql.