From The Document Foundation Wiki
- Create your own background:
- Select the slide where you want to change the background
- Open the Master Slides deck
- Right-click anywhere within the Used in This Presentation category
- Select Edit Master
- Select ▸
- Change to the new master by clicking it in the slide pane
- Right-click on the slide and select Set Background Image for Slide
- Select your background image and click Open
- Select ▸
- Left-click on the newly created Master Slide (NOTE: The selected background image will not load into the preview until it has been clicked) within the Used in This Presentation category
- The created Master Slide can be reused for as many slides as needed