1. open blank document
2. click on empty space document that you want to add background image
3. Top right have 2 Tabs : Format and Document : Yes you click on Document
4. below these tap you will see 3 Tabs : Document Section Bookmarks. : Yes you should click on ' Section '
5. under Section tab : you will see Background section and Dropdown selected ' No Fill '
6. Select it : and choose ' Image Fill ' at dropdown menu
7. At last select [ Choose... ] will show pop up window for choose Image that you want and ' Open '
8. Finish : It is Background image on this document.