I have a webhost who is running PhpNuke 7.4 for me. Also included is Coppermine Photo Gallery.

What block do I need to allow photos to be randomly shown in the center, and can I specify what photo albums the radom pics come from?

http://modelwarships.us/php

It comes as part of the coppermine package - it's called cpg_random_block i think, can be found in admin>blocks>new block>found in the file menu. Then specify center if thats where you want it.
I dont know much about copermine so i dont know wether you can specify which galleries it uses.