I always use HD 1920 x 1080 backgrounds, then scroll to 960,540 at start of layout so it is exactly in the middle of that 1920x1080 area, so if a game is in landscape mode and game widow set to 960 x 540 or 800 x 450 ( both 16:9 ratios ) there is plenty of stuff on all sides, so no bars will show.
Works in portrait mode too, so if your game window is 450 x 800 you have 140 pixels of background on top and bottom. If you need more just adjust your background to cover more.
If you do not use one large background image, then just fill it with some objects which I believe is what dop2000 is saying.
Use scale outer of course and you have to put all your stuff in the middle since you are scrolling to 960,540, you can draw a rectangle the size of your game window and place it at the bottom layer at 960,540 position as a guide