Once you've applied a menu template to your project, you'll automatically go to a customization screen. If you click to select the background of your template in the Disc Layout monitor, you'll see the option to replace the background with a still, video and/or audio.
I walk you through the whole process of customizing your menus, step by step, in my books, by the way. They're available on Amazon.com (where they're getting great reviews!) and at the Muvipix store.
Select the image in organize and drag& drop in disc menu layout.