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.
