Okay I found the pictures in the following directory
~~\Rome - Total War\Data\world\maps\campaign\imperial_campaign

Now it has 2 generic maps:
radar_map1 and radar_map2

The description for the 3 roman factions are in the dir as well (description_julii.txt; description_brutii.txt, description_scipii.txt), but not the others. Not even the unlockable ones.
So we need to find the txt which references which picture to use and the txt with the descriptions for unlockable factions.