Windoze or Nix?
If Windoze... for shame and I could not help much there as I forswore it years ago.
If a flavor of Unix, things should be working just fine as GD2 is included in every distro I know of. No pointing to worry about there.
I am assuming you have gone to CP>Registration Settings>Basic Options>CAPTCHA verification? and see that the drop-down select has "Use GD2" as an option?
Beyond that, I'd need some more detail about your environment to be of any use at all. Are you on a hosted account? If so, who? What is the OS and what kind of configuration control do you have?
If on a hosted account, you'd get best results with their customer support folks as UBBThreads has nothing to do with what graphics library is on the server.
