ProVoc backgrounds are essentially Quartz compositions (.qtz files). Quartz compositions can be easily found on the internet, such as from www.quartzcompositions.com. To choose a new background:
Quartz compositions can easily be edited without writing a single line of code with Quartz Composer (part of the Developer Tools available from your Tiger Install DVD). Read the documentation from Apple.
If you want the background to react to user actions (such as making special effects for correct or wrong answers), download the ProVoc Background API.