When there are two or more regions overlapping on the stage, the depth visibility is dictated by the track order.  The contents of the first track will always show on top, whereas the contents of the last track will always show up on the bottom of everything else on the stage.  To adjust the track order, select the track you want to move, and use the up and down arrows above the track list.  For example, something you want to display upfront should be in the first track, while a background image should be in the last track.