1. Go to Page Background (or Block Style)
The page background section can be found in the Settings menu on the right sidebar:
In order to get to the block style background, hover your pointer outside the editable zone of the page, and click on the block style button:
2: Select or upload an image:
3: Set the background image to Stretch from the menu.
And that’s it. Now your background image is responsive, adapting its width to your visitor’s screen size.