When you introduce the background image as a Storyboard element of its own, it'll become its own storyboard object. For a storyboard object, after its last command is finished, it will unload itself. So most likely this is what's happening here.
You'll need to have some sort of command at the very end of the map to keep that background visible. You can probably do some sort of fade out at the very end or something and that'll be good.
Best of luck!