Right now, the main menu background is from whichver culture you played last (greek, egyptian, norse, atlantean), which makes adding a custom main menu background scenario a bit of a hassle.
My suggestion is give us option in the settings for:
-Last played culture
-Random (from any in the “background” folder)
-Select (which opens a window for you to select one specific file to be your background)
That way you can have more than one main menu background mod active, as the community (including myself) have already made a few, and swapping between them requiring game-restart is really disruptive UX.