In the section -- Add Still Image to the Background -- this help page states:
"You can include still images in a menu as part of the background."
This is a bit misleading. All Encore menus are Photoshop PSD files. An Encore menu background is a Photoshop background layer. It can be only a single layer. It can consist of multiple images depending on how you set it up in Photoshop. But adding images to a menu from within Encore does not put them in the background layer. They are separate layers in the menu's Photoshop file and essentially "float" above the background. Just as graphics and text are separate layers.
To clarify, you can include images in a menu, above the background layer.
