I was getting too much comment spam from KnowledgebasePublisher (aka KBPublisher) and decided to enable visual confirmation codes (captchas). I run KnowledgebasePublisher on a Windows server with FastCGI.
The captcha images were not appearing after I enabled the feature. What I had to do was enable the GD2 extension in PHP.INI and restart IIS.
Hope that helps if you have a similar problem. This is mostly a reminder for myself if I encounter it again in the future.