The following covers were contributed by NoteTaker user D.R.
To cover an active notebook file, use Option-Cmd-K or the Cover command from the View menu. Drag and drop an image or a custom cover graphic on the cover. To format it, use the Background Image sub-menu from the Format menu.
If the Toolbar is visible, double-clicking on any blank area near the middle of the Toolbar itself will also invoke the Cover command. Likewise, double-clicking on a visible cover will uncover it.