To add an Image to the background of a new section, first, click +ADD SECTION.
Next, 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. However, if you want to add a new image, you can either click on+ Upload an Image to upload one stored in your computer or click on + Get free stock images if you want to select one from the stock image library.
Set whether you would like the parallax effect applied or the image set to full width by ticking on the Parallax effect box and the Full width background box.
For more information on the Parallax effect, please see this support article.
When finished, click Save.
The image will now show as the section background.