Are you asking if you can set a unique custom image for their clothed and naked/sex scene state? If so then the answer is no. As for the "switching to the reactive one" behavior, its not exclusive to the player. There various scenes in TiTS JS where NPC portraits become blank, I'm no expert but perhaps the game is calling for the image incorrectly and errors as a blank image.
Anyway, it would be cool if there was a way to set an image for the different states of the PC and Bess/Ben, perhaps in the future the team can implement that or someone from the community can create a TiTS JS mod.
