DeutschRitter, the additional factions data go in your Factions folder, which would be in something like My Documents/My Games/GalCiv3.
In Factions, you will find a) four sub-folders and the custom faction files.
The four sub-folders contain the custom art in ".png" format as follows a) LeaderBGs: backgrounds (1920x1028), LeaderFGs: large portraits (1920 x 1080) c) Logos : logos(!) (128 x 128) and d) Portraits: small portraits (128 x 128).
So when you download materials, you need to move the downloaded content into the right folders of your Factions folder group. The faction files themselves go directly into the Factions folder, they don't have a subfolder of their own.
When you start the game, the game reads the faction files (i.e. ".faction") and these are the custom races that will appear in your main screen, ready to select. As well, the art in the various subfolders of Factions can be chosen by the user when they wish to make a custom faction of their own. As an example, you will see that in my download I added in Factions another folder, called Unused, which the game has no use for. This folder contains alternative Human art that you can move into LeaderBGs and Portraits so that if you want you can either make a new faction with this art, or replace the Terran Alliance art for a change.