In the documentation for GeoBackground there is an example of an image used as GeoBackground, with the following syntax:
GeoGraphics[{GeoStyling["ReliefMap", Opacity[0.8]], 
  Polygon[Entity["Country", "UnitedStates"]]}, 
   GeoBackground -> 
  GeoStyling[{"Image", 
    CountryData[Entity["Country", "UnitedStates"], "Flag"]}], 
   GeoRangePadding -> None]
This may help. You may also consider Overlay.