Click Edit Slides.
Upload an image or set a color for the background of your slide.
Set your background position and size. See here to learn about background image sizing.
Choose whether or not to add layers (text, buttons, images) to your slide.
When a layer is selected, you will see placement and design options below your main slide:
Click Add Slide repeat above steps as necessary.
When you’re happy with your slider, scroll to the bottom to save your settings.