I think I figured out the problem. When I donwloaded the Display Demo from Itch (on my Windows PC), some of the file names got chagned. For example, I have a file called "!$_Boat.png_" but the Display Demo is actually looking for a file called "!%24_Boat.png_" instead.
I can resolve this issue by manually renaming all the image assets in the demo's project folder.