Okay I'll give it a go.
As far as Backgrounds download any background/wallpaper png file that is 1920x1080 and place into folder
C:\Users\[Your Computers name]\My Documents\My Games\GalCiv3\Factions\LeaderBGs
Foreground pictures works best by cutting out anything else associated with the picture other than the image of the leader itself. One can use many programs to do this one is Adobe Photoshop which you can get a 1 month free trial, or there are other free web based editors out there. This picture also works best as 1920x1080 .png file. Once this is done place in folder.
C:\Users\[Your Computers name]\My Documents\My Games\GalCiv3\Factions\LeaderFGs
Leader pictures should be cropped from the previous imaged and reduced in size to 128x128 .png file, then place in folder.
C:\Users\[Your Computers name]\My Documents\My Games\GalCiv3\Factions\Portraits
Finally Logos, any icon\logo image in .png will work again should be size 128x128. Once you have a file place that in the following folder
C:\Users\[Your Computers name]\My Documents\My Games\GalCiv3\Factions\Logos
Last start the game and they will appear for your custom factions. Note if you already have the game running you will need to restart it.