You will have to edit Descr_Cultures. Find the section for the Carthaginian culture and change the portrait_mapping value to Carthaginian. This is assuming that you have created a Data\UI\Carthaginian\Portraits folder and its subfolders to put your your new (and the old) portraits in.

Alternately you could just put your new portraits in the Data\UI\Eastern\Portraits folders. But then the Eastern culture factions will have them too.