When an element is selected, two grey arrows appear in the bottom left corner. These let you control the front-to-back ordering of this element. Be careful moving the element backwards because it will become harder to select in the general Edit mode, though the Selection Rotator tool makes that process simple.
Along the bottom towards the right, you will also see the “Hide Other Elements” switch—a means to show only the current element and the background picture, thereby making it easier to line up that element with features in the background. This is particularly useful for masking out foreground elements.