This is what I'm using.
//Add Artwork: System Flyer, x,y Co-ordinates height and width
local sysfl = fe.add_image("/Images/Systems/[Title].png", 100, 500, 780, 350);
For a background you'll either stretch the background picture (e.g. 0, 0, 1920, 1080) or if picture is already at proper resolution add it it without any co-ordinates (0,0,0,0). Plus if you have opted for not showing brackets in lists then [Title].png also won't include brackets.
[I have a separate layout.nut for dispaly-menu, so calling [Title] from there lists consoles/systems]