If this is the result you want and the file has already been flattened:
Make a rectangular selection that includes the frame, but not its drop shadow.
Inverse the selection
Add a blank layer below the image layer and Edit > Fill with light gray
Set the Blending mode to Multiply
Deselect the marching ants.
Please post a screenshot (pre-flattening) with the pertinent Panels (Layers, Channels, …) visible.
norman sanders, yes that is what I want. Thank you!
Happy to be able to help. Good luck!