Ethan: th09_msg was an early tool that could turn the msg files into semi-readable txt files and revert back. It worked for a bunch of games, not just PoFV.
Turboo, if you use thmsg, you can find entries starting with 2, like "2,0,5" and "2,1,1". These control the portraits; 2 designates portrait selection, 0 or 1 designates which character to change, and the last number corresponds to the portrait number.