That's actually the most useless captcha ever.

It basically doesn't work. It's security mechanism is that the submit button is disabled until you've drawn the figure. That's it. A spammer would simply do a direct POST to the server, completely bypassing the system that is to prevent spamming.

A real captcha stores the characters server-side (in a session variable) and sends the image to the client. Finally it expects the POSTer to submit the same value as it has stored in the session variable. And that is a proper way to deflect spammers.