Check your game folder. You'll find a subfolder "portraits" in subfolder campmap, iirc. The "portraits" folder itself is divided into cultural/religious groups. The ones you seek should be in Vikings. You can simply copy those to another folder but you have to make sure that they're named correctly. For example, Catholic/Assassin/ should contain portraits that are named Assassin001 up to Assassin030 or something. You'll see what I mean when you look at the folder. Make a backup copy of the folder before you change anything and then test it. I'm not sure if the changes affect savegames, you may have to start a new campaign.

R'as