How can I add security on new user registration form

This question is overly generic. Please review this:

https://codex.www.ads-software.com/Forum_Welcome#Include_as_much_information_as_possible

I like to know how to activate or put security image such as captcha code to help block spam emails. I would like to add it on new user registration form or page. Hope you understand my question.

Here is the page I am refering to, where I want to add security image registration form. The problem is I keep on receiving spam e-mails.

Thank you, I found a solution.

Sharp!