That said, there are some oddities that suggests this feature is not fully baked. For whatever reason my wallpaper vanishes if I enter a NAS webpage via its IP address but if I use the host name the wallpaper appears again.
Either I didn't want to download a huge image as a background or that I read somewhere that the image should be limited to a maximum dimension. Whatever it was, I have a set of backgrounds that I nicked from Mac OS X/macOS desktop pictures and then reduced the big ones to 3840x2160. Forget what I used to batch process them.
I use just a small "logo" image on the login screen (along with a custom name) and then a simple color-specific background to know what nas I am using. color is a match to my logo image, so I mind-map that way