Something I started doing for myself and figured I'd share my progress:
Included are PNG (for custom race) and also DDS (so that you can overwrite the main files). I recommend backups and remind people that they are 100% guaranteed to be overwritten any time there's any kind of patch.
To overwrite the vanilla civs, the PNG files go into the gfx\factions\logos folder and the DDS files go into the gfx\civilizations
If you choose to just use the logos for custom civs, put them in the user\mygames\.... folder. You will probably need to rename the files though.
Note that the folder for the Snathi PNG file is in the DLC folder in the main directory: GalCiv3\DLC\DLC3_Snathi\gfx\factions\logos.
Download here