I think it's clear that software sold in a physical package, or even download-only software presented with a digital mock-up of box art, would make physical media scans/official representations of box art the highest priority.
But, in the case of download-only software with no such images, all you can really add is a screenshot of the program. I wonder, though, if such programs boot with splash screens, is it acceptable (or even preferable) to add the splash screen, and to present it first in the image order?
Apologies if I'm retreading old ground. Searching is difficult here, but I have to confess to my own laziness as well.