AMake sure "Use BG Image" is checked. For "Image Source" select the general input method of either 'Use Drop In' or 'Use Resources/Warehouse'.
For Drop in, you will be shown an image well, in which you can drop an image.
For Resources/Warehouse method, the "Image Folders" selection will be shown and then select the specific method of either 'Use Resources' or 'Use Warehouse folder'. Either a 'Resources Image' or a 'Warehouse Image' input box will be shown, in which you can enter the path to the image.