First, to enable Captchas you need the captcha module. Don't get fooled by the documentation stating it requires textimage module. It doesn't. Instead it serves simple, text based captchas. I personally prefer text based captchas over image based, since they are a lot less error prone, and they are accessible to (color)blind people, too. Additionally, as I will explain later on, image based captchas can as easily be bypassed as text based ones, so they don't provide additional security.