Captcha for Adding question in Q&A

Team,
I would like to keep the permission to post question and answer open to guests/QA guest. This means even without registration they should be able to post question/answer.

I needed a captcha for this. Obviously captcha in registration/login would not work here.

I see there is a securimage folder in the QA plugin. Since it is related to captcha, I am wondering where it is being used.

Can you help me out?

Thanks
Thomas