Hello.
This it the guide to add captcha:
http://www.openscriptsolution.com/2009/09/11/how-to-add-captcha-security-code-in-osticket-system-1-6-rc5(http://www.openscriptsolution.com/2009/09/11/how-to-add-captcha-security-code-in-osticket-system-1-6-rc5)