From The Document Foundation Wiki
How do I use a drawing made with LibreOffice Draw as a background?
To use a drawing made with LibreOffice Draw as a background for a presentation, you must add the Draw image to the Slide Master:
- Create the illustration or image in LibreOffice Draw, within the master ▸
- When your illustration is complete, select ▸ ▸
- Select the category (Presentations to make it available to all users, My Templates to make it only available to you)
- Click Save
- Switch to your presentation in Impress
- Click ▸
- Click Load
- Browse to the Category where you saved your custom background
- Select the Template
- Click OK
- Click OK again and the custom background created in LibreOffice Draw will be added to the Master Page of your current presentation.