Ran in to a couple problems working with CAPTCHAS on Fedora Core 5. The problem was simple. PHP-GD was not installed. So, just run:
yum install php-gd
This should give you the correct package to be up and running with building in extra security into your web forms using captchas.
Wanted to update this with another issue with php-gd.
If your Apache error log shows that imageCreateFromJPEG() was undefined and machine runs CentOS 5 which is a community supported rebuild of Red Hat Enterprise Linux 5, then you’ll need to install it. CentOS and RHEL do not install the GD graphics library support for PHP by default.
So a simple
sudo yum install php-gd
sudo /usr/sbin/apachectl graceful