OK, I've done that and when I update any of the image fields, when the upload is complete and the results page comes up, whatever images were in the database for that listing have been wiped.
I assume that somehow I need to call whatever was in the database for that listing to show in the field where there are images so that I can leave whichever ones need to be retained but I'm not clear how to do that.
Revised page attached, hopefully with correct code this time.