It's some proprietary image format... currently can't be modded.
The file to look at would *probably* be Space01.tiles.
The raw files are probably targa (.tga) like the ships, but then again what do I know

It's all geek to me LOL
If enough people ask for it and it's feasable (without slowing the engine to a crawl). Stardock might change this in a future version to allow us to alter a .pcx or .tga file in the Gfx directory.
PS - and for the record I vote for this! Even if not... maybe Stardock could create a new one for the expansion pack with a different color and feel to it (maybe move away from blues and try dark greens or reds for the expansion pack) The various colored nebula behind the races pics look especially cool! Just please not a multi-clored rainbow of stars