Background images must be at a maximum of 1024x768 pixels for 4:3, 1366x768 pixels for 16:9. Although 1366x768 (16:9) is recommended, most background ratios that are 4:3 or 16:9 should also work reasonably well (for example, 800x600 or 852x480). Using an image with a different ratio is fine, but will result in letterboxing (black bars at the top/bottom of the screen) or upscaling (resized image to fill the whole screen).
I suggest this to be changed to max 1920x1080, it doesn't hurt the image size that much.
Back when this rule was created 1080p monitors weren't that common, now we see more and more 4k monitors.
Change it to:
Background images must be at a maximum of 3mb (3072kb) in size. There are no height:width restrictions. background ratios that are 4:3 or 16:9 are recommended. Using an image with a different ratio is fine, but will result in letterboxing (black bars at the top/bottom of the screen) or upscaling (resized image to fill the whole screen).