Background Templates for Presentations

When i create a presentation, i only have 3 templates i can use. How do i create my own background template that i can apply to the presentation without using the Still & Motion bckgrnd.

Design a slide, then right-click the thumbnail of that slide from the right slide deck preview > "Save as Template."

Thanks but i was wanting to create my own background images for templates that are like the existing template for presentation. if i could attach an image here i would but i'm not able to

You can include images in your slide templates by adding a media box to the slide