I have successfully changed other NPC portraits by simply editing the portrait.mes file. However, I cannot find a entry for "riana" in that file. I'm guessing it's a default protrait as other "working ladies" in the brothel have the same portrait. Where is this portrait? What is it's name so I can find it in the mes file? Anyone know? Thanks for your help!
Since she was taken out of the game it's probable that her portrait is more like a placeholder. The best way to change it would be in protos.tab imo.