To add an Image to the background of a new section, first, click + ADD SECTION.
Alternatively, to add a video to the background of an existing section click the section's Settings Cog on the left-hand side of the section.
Click on the Background Tab.
Choose an Image already uploaded into the Images area or select + Upload an Image to upload a new image.
When finished, click Save.
Your Image will now appear in the background of the section.