No problem.
If you add the four images to the portrait folder, you go the 'character' or 'inventory' screen in-game (can't remember which), and beneath your image there's an option to change the portrait.
Make sure the files have the right titles when you add them to the portrait folder too. An example would be:
Large color portrait = human_x1_lg.png
Small color portrait = human_x1_sm.png
Large watercolor portrait = human_x1_convo.png
Small watercolor portrait = human_x1_si.png
Hope this helps!