Steam

That looks like the horizontal grid. All the custom images are in a folder in Steam (see below for the location). You should be able to copy an image into that folder, and as long as it's using the correct name format, Steam should use that image just fine. You'll need to know the game's ID, but you should be able to figure that out by cross-referencing other custom images

• Image folder location: (steam directory)/userdata/(user id)/config/grid/

• horizontal grid format: (game id).(file format) (ex: 2284631436.png)

• vertical grid format: (game id)p.(file format) (ex: 2284631436p.png)

• hero (aka wallpaper) format: (game id)_hero.(file format) (ex: 2284631436_hero.png)

• logo format: (game id)_logo.(file format) (ex: 2284631436_logo.png)

• how to find game id: there's no easy way to find it, but you can set a custom image for the game with any of the other options, then look into the folder to see what the custom image is called. You then should be able to figure out what the game id is by comparing it to the name format

Just to be clear, do you mean the Steam game "art" (or whatever you call it)? https://www.steamgriddb.com/ Stream grid db might be an option, if so. It involves logging into their site with your Steam account, and downloading SGDBoop, though. https://www.steamgriddb.com/boop

The simplest way is to launch the game, then go back to your steam library home. You should now have the gray image there. Then you can just rightclick and there will be an option to select custom art.

I know that I've done it. I think it was Boop. Honestly Steam frankly sucks for this and I've given up trying to making it look good. Tried GoG Galaxy's outside launcher integration, gave up on that too, and I've been really pleased with Playnite for many years.