First: You will be working in the following folder (I first work in multiple saved folders and the transfer them over when ready to use), "C:\Users\K****** B******\Documents\My Games\GalCiv3\Factions". You will see four folders named: LeaderBGs, LeaderFGs, Logos, and Portraits.
Second: I use the following two free programs to edit and adjust images: "GIMP2" (go to YouTube to see how to cut out images), and "paint.net" (easier and quicker to use, just not as strong, in my opinion).
*** note *** make all images in PNG
Third: Choose a background you want to use, crop it and resize it to be 1920x1080.
Fourth: Choose a foreground you want to use, cut it out, resize it and put it on a new 1920x1080 image that you have already added a second transparent layer and deleted the first layer on (it will be right if the complete blank is covered in a checkered pattern). **** note **** when you transfer the new image onto its 1920x1080 blank it should not go all the way top to bottom, bottom to about 80% up works about the best.
Fifth: take your FG image copy it and crop it so that it is at 250x250, save this as your portrait (unless you want to use a different image for a portrait).
Sixth: find the logo you want to use (or create your own as you did with the FG), size and crop it to 128x128.
Seventh: Put all four pictures into their correct folders in the faction file.
Last: Start up GC3 and make a new race using all the images you have setup.
This is only a quick guide, but should get you started in the right direction I hope, thanks.
*** finale note *** All the sizes I gave or what I use for constancy, and work well for me.