I am also very confused about this.
Here's what WindowBlinds claims:
As you can see, I've set an image for 125% scaling. This image is very different from what it should be, it's there just for testing.
Here's what in the corresponding dpi file:
(I have no idea why 125% scaling corresponds to 120 value in the file).
I'm using Windows 10 Home with 'make everything bigger' set to 125% and as best as I can tell, setting High DPI image has no effect whatsoever. It still uses the base image and stretches it horribly for 125% zoom.
Am I doing something wrong or is this broken for everyone?