You cannot do proper splashscreen without specifying a bunch of splashscreen for different screen ratio.
There is actually a lot of option to customize the loading screen.
  • iconUrl
  • exponentIconColor
  • exponentIconGrayscale
  • backgroundImageUrl
  • backgroundColor
  • hideExponentText
The fact is backgroundImageUrl doesn't allow to have a set of splashscreen, and pick the necessary one, given the ratio.