Load up the PCX files you find in the GalCiv directories. In you paint program and then check the file properties to see what size and resilutions they are. Then stay in those boundries and you are very free with content.
As for the data files they are in the DATA directory, and as far as I can tell they all can be MOD'ed via the simple Notebook program that comes with windows. XP = Start Menu -> Programs -> Accessories. As for how to do it. Look to BoogiBac's tutorials. Also it would be a good idea to copy the directories you will be modding and practice on the copies first. Good luck. And feel free to ask more questions. Try to do some fiddling around with swapping music tracks, Led Zepplin or Bach works quite well.