Lectora's Layering Order
Lectora's objects are layered according to the order of the left-hand pane, starting from the topmost object. Therefore, the first image beneath a page in the left-hand pane will be the furthest back, since it is the first to show.

The last image beneath the page (in the left-hand pane) will be layered above any image shown before it. To change the layering of objects, simply drag-and-drop to different positions in the left-hand pane or use the layering option within the right-click menu.