When someone wants to register, the captcha doesn't show. So the picture doesn't show but people are asked to fill it in. No matter what they do, it won't work. What can I do to restore it?
You have to make sure your php has been compiled with the gd library installed and that the php version is compatible with the captcha code, just make sure gd library is installed first and see how you go
And where do I do that? There are no other options with the Human verification options.
The native vBulletin human verification methods meet with either spotty success, or don't work at all. And that's not just vBulletin, but all the major email providers as well. Check the link in my signature when you're serious about stopping spam.
Captchas and other such gimmicks, puzzles, photo identification and so on, were defeated long ago. They're obsolete.