How To Change A Section/Container’s Background Image
Click on the Section Settings handle

to enter the Section Settings
Go to Style > Background > Background Type > Classic
Under Image click the + sign and choose your image.
How To Change An Entire Page’s Background Image
Click the Post Settings icon in the upper left of the Top Bar.

Click the Style tab.

Click the pencil icon next to Background Type.
Under Image, click the + sign and choose your image.