Perfect! Much appreciated, Larry.
It actually depends on how you select the image. Usually, one uses the black arrow -- but then you select the container frame for the image. The 'image' is, indeed, the first graphic inside this frame.
If you select the image with the white arrow, your original script will work.
Why does one get the "Display Performance" option with "the frame" selected? A little experiment shows that you don't get that with a 'regular' drawn graphic or text frame. Now place an image as usual, and you do get the option. Select the image -- not the frame -- and delete it. Now you can set the 'Display Performance' of an empty frame!
This gives you an important clue, though. The empty frame is of the type that displays a cross inside, and its default contents type is "Image". That's the clue that ID uses to allow the display options.
Thanks Jongware. It seems odd to me though, that the InDesign DOM has localDisplaySetting as a property of containers (rectangles, polygons, ellipses), when changing this property has no effect on the performance display setting of any item in the doc at all. It makes sense to me to only have localDisplaySetting as a property of graphics. But anyway, I got it to work -- that's the main thing!