There is no special changes or requirements for larger background images to function for PChat.
Just so you know it does not have to be 800x600. The size is completely arbitrary. Though i don't recommend going to large because some users have small resolution monitors. If you want to be compatible with the largest number of monitor resolutions including small netbook resolution of 800x480 (which some people actually use believe it or not). Then 800x450 is the safest background size.
On another note, the linux pserver plugin called soundlimit has been known to cause problems with being able to move around on larger backgrounds, limited all types of movement to the old 512x384 background size. Removing this server plugin is recommended, and to be completely secure make sure the latest Valcheck pserver plugin is installed. The Soundlimit plugin is obsolete.
The official forums for the PChat client software is located
http://pchat.org/forums/
