Well, when I first installed BKB Mod, the portrait for every King changed each time I closed the game and opened it later to play. After about 4 times of this, it settled on portraits for each King.

I did this again with a couple other Kings (changing their portrait with above method) and it worked with them too. The portraits being used by the Celtic faction Kings were all in the generals folder (vikings/catholic/generals) instead of Kings which I also found odd. But I changed them to more preferable ones and it worked with a simple number switch. Perhaps the mod installed in a peculiar fashion, but everything works fine and there are no other irregularities, so I don't really care.