1. if you want to change a background for the entire landing page, you must go to the layers panel on the left side of the editor,
2. Then, double-click on the first "Page" icon (= Body) until you see the "Page settings",
3. Activate the background for the page by clicking "ON" on the ON/OFF bar. Button "Change" (1) lets you change the background image if an image has already been used in the background. If there is no background picture on your landing page, then you need to use "Upload picture" button. Instead of the image, you can add a "Gradient" to the background (2).
4. After clicking on the "Change image" / "Upload" button the gallery will open. Please select the image, which you want to add to the background. If the gallery is empty, you must upload an image first, and then choose it,
5. After you click on an image in the gallery, the background for the page is set,
6. You can set the location and behavior of the background image individually, using extra options in the "Page options panel",
TIP: You can upload the image on the page background and then set the gradient background for a single section to achieve such effect as below.