Good suggestion with the display picture command. I can achieve the same results with it but unfortunately, in my case I am using Irina's Visual Novel Busts Plugin that has an issue with layer order when paired up with the Show Picture commands. I also extensively Galv's layer graphics as a separate "Background Picture" folder (for organizational purposes )as well because my Picture folder is now at 500+ images and it's getting hard to navigate what I need whenever I need a specific background image for a scene in an event.
It might be easier to see if someone can edit the Bust plugin you're using... basically you need the bust to show above pictures but below the message window? I'm not familiar with that plugin, but there should be some sort of way to change the Z layer it's drawing the busts to. Alternatively, see if any plugins exist that change the Picture Z coordinates (not sure if this exists yet). Good luck!