Which Steam art files your game has, and every social and store banner size cut from your key art.
Sizes are Steam's current upload sizes. Library art only shows after you upload it in Steamworks → Store Page → Graphical Assets → Library Assets.
Upload your biggest key art (ideally 3840 wide, without text near the edges). Each size is center-cropped; slide to move the crop left or right. Runs in your browser: nothing is uploaded.