How can I set an artwork for this?

[Arnulf]

Screenshot

I know how to change the artwork for the box art, and the banner/capsule, logo, and icon. But his one here eludes me.

I would like to know where in the file system I have to put the image and the dimensions needed (and maybe type, although I guess PNG or JPG would be fine)

[WarnerCK]

I use SGDBoop from the SteamGridDB to automatically set artwork for non-Steam games.

I think that one is the library header[partner.steamgames.com].

[MarvinR]

in desktop mode, right click it when it's in the last played slot and choose artwork.
or, as mentioned above, boop it with sgdboop.

don't know the exact dimensions.

[nicoper]

Custom grid artwork goes in Steam\userdata\<SteamID>\config\grid
You can also change that one by right-clicking, then choosing the appropriate option and selecting an image file. I believe that one is just stored as <appid>.png/jpg in the grid folder.