As far as I know - replace exisiting ones.. That's what I did here - every last one of them. The BMPs are associated with each of the various Mod files in the space folder.
Some of the stock ones where not that great, I thought. I won't miss them. To add new ones they would have to be associated with one of the space.mod files in the space folder.
I am working on an SFC3 version of this, by adapting James Formo's SFC2 Scenery Mod. I've been working with BC for a few months now making systems, and I want those graphics in my SFC!