Portraits stored in STI format. There is a STI editor somewhere.
You need to extract portraits from the SLF archive into the appropriate place and then edit portraits with STI editor.Sounds simple... but could take alot of efforts if you are not familiar with that.
Also I believe you cannot ADD additional portraits, you only can modify existing ones. That's because all portraits are hardcoded into the game. This is true for NPCs, but for PCs this may be wrong (depends on how portraits stored in the save file: by ID, or by NAME).