Right now, I use both captcha and security questions.
Security questions are things like "Who is Rin's twin brother?"
There are several questions, every user gets one at random (on top of the captcha). But even then, bots will get in after a while, so once that happens you have to go and change the questions.
I wasn't using questions yet when you signed up.
I hadn't heard of captcha with images before... I don't think the forum as it is supports them.