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We had a similar issue with comics and we ended up coming with something that would scale your image to fit the cover while preserving the aspect ratio, and adding a blurred background behind it. You can see some examples here:

If you think that's a good solution I can enable it for the physical games section.

It looks like the bulk of the comics on the first few pages opted to resize to fit itch's formatting, but for those physical game creators who don't want to do that, the blurred background is a good solution.