UBB.threads PHP Forum Software Community Forums UBB.threads Installation and Upgrade Support Captcha image missing

I have to take back my previous observation on the captcha test.
I had captcha disabled on my forum, so users can registry.
I enable and then I got a broking link sign on I.E.
and a blank page on Firefox.

If I click on the broking image, it shows no file name, but the PHP code.