Data\ui\ROMAN\INTERFACE\SHAREDPAGE_01.tga
and
Data\ui\ROMAN\INTERFACE\STRATPAGE_02.tga

You have there the symbols for all the factions. Simply paste the new symbol on the old one.

I am not sure if the very small symbol below the settlement changes in this way.
But it is sure that it changes the big ones in the two posts, as well as the symbol in the diplomacy pannel and the small one in the prebattle screen.

Cheers.