From The Document Foundation Wiki
How to insert a logo or a graphic into the header
- Go to ▸ .
- Click on the Header tab, then on the More button.
- In the window that opens, click on the Background tab.
- In the listbox, choose the type: Graphic.
- Finish by setting any necessary parameters and adjustments, particularly the height of the header that will give you a correct display and printing of the logo or graphic.