Don't know as Im only a newbie, :o)
BUT have you tried using a square plane from your primatives and applying the background image to that and positioning it just where you want it, as it's an object it should accept the shadows
...i find that better than using the background for floor as sometimes things arent where you want them to be

Tools:- Win10, Dell XPS8900, ZBrush, Marvelous Designer 11, Hex 2, PSP8. PSP 2019 Ultimate, DAZ Studio, Affinity Photo, Affinity Designer, Filterforge 11, flowscape,  Classic UVMapper, and several headache tablets.