This is orientation issue. Most likely, on smaller screens, the content height is higher because as the screen get smaller, content tend to stack up on top of each other (this is how responsive works by default). With longer content, background image set on the section will try to stretch to cover entire part. That is how background image work. See this thread:
After understanding how background image work, the easiest work around here is to use HIDE DURING BREAKPOINTS under Customize tab. Instead of adding the background image on section, add it on ROW. Then create 2 rows. The first one is hidden on mobile screen and the other is hidden on desktop screen. Use different background image for each ROW. For ROW that is set to show on mobile, use a vertical type of background image that will fit. Something that is longer.
Hope this helps.