Discovered a bizarre issue with shop menus: If you leave one without a concluding message of some sort, it causes the bust portrait to hang around until you start another conversation. This does not affect ordinary conversations, just exits from the shop interface.
Thankfully, I have a good idea why: the shop interface has been massive customized, and the built in image flushing of my faceset to bust script does not take that into account, so it's own automated image flushing code does not take effect unless I exit the shop window to another dialogue scene, THEN exit that, at which point it works properly.
However, since that's just tedious, I'm going have to do the still tedious but slightly less irritating option of just adding an "Erase Picture" command at the exit of every shop window in events since the automatic scripted version does not play nice with my modified shop window code.
This works nicely enough and have seen no issues from it, so I'm going to go with that unless I can patch the shop code somehow.
Also, I don't NEED a script to do bust portraits: I could just manually add show picture commands to dialogue scenes with X/Y coordinates to place them, but I'll be blunt, that's even more tedious, so have a script do most of the work is easier, and the bug I encountered is minor and easily worked around, so I'll go with that.