Hi Naved,

both a mathematical as well as a graphical captcha are already included in the forum. You may activate one of them in the admin settings for different purposes (e.g., registration, contact form, posts of unregistered users only, etc.). What are your specific problems?
BTW, a captcha should only be used as a last resort if none of the other methods (IP blocks, Akismet, Bad-Behavior) are successful.