How responsive images works is it display the image max-height OR max-width to make sure there are no gaps. Since there are a wide variety of monitor size and device shape, the background need to change in order to fit to different shape and size of view port. This is the definition of responsive in which, the plugin will detect the monitor size and shape and “respond” to it by adjusting itself to fill the entire space without gaps.
Because of how responsive background works, the background image can be cropped in smaller monitor size to ensure the entire monitor will be covered by the background image. You will need to try different background image SHAPE to see which one will work for you.
The rule of thumb is if you want the heigh of the background to remain relatively the same, use a rectangular image. If you want the width to stay relatively the same, use a square image.