Putting overlays behind the game?

I was wondering if there was a way to put an overlay behind the game? Rather than on top? (Specifically I’m on windows, but I assume the question works as General).

I know "overlay’ means on top, but when using the ‘core provided’ aspect ratio setting, most cores have different aspect ratios. Being windows, I use overlays for borders rather than touch screen buttons. You can set pre-core overlays which mostly works, but there’s still the issue of Arcade. There’s so many different aspect ratios for Arcade games. Street Fighter has a different ratio than Donkey Kong. Donkey Kong has a different ratio than Ms.Pac-Man. Etc. I think the whole thing could be fixed by having the overlay behind the games. Then I could just use a large wallpaper type image to cover the whole thing, and the game would be on top. No blank black space.

And if this isn’t a thing I can do, I guess that’s a suggestion. Or a separate Background option.

I think I saw that done in an older version. I don’t know how or if it still possible though. I would love to see this, and a way to change the background per system or game even. It would be a great addition to a RetroArch / Hyperspin system.

I think you can use shaders for this purpose