Red Hat Fedora SELinux Policy
It appears that starting with Fedora Core 3 SELinux is enabled by default and the Apache daemon is protected, meaning the httpd web server daemon cannot write files. For Qdig to be able to write converted images and / or empty caption .txt files the policy must be adjusted.
One way to do this is by installing the system-config-securitylevel package (yum upgrade system-config-securitylevel) and then using system-config-securitylevel to disable SELinux for the Apache httpd daemon. (Thanks to kevinmc2000 for confirming this solution via the Qdig Help forum.)
If you have suggestions for making this page more helpful, let me know. -Hagan