Access the background options by clicking on the Page Background button found on the right sidebar:
These additional options are shown below:
Background image. This allows you to add/change any image to act as the background of your landing page. Click on the image icon to upload a new image from your computer or add one from a link. You can also change or remove the image with the buttons directly below it.
After you add an image, you will be presented with extra options to customize your page background further. You can set it to Stretch, and now your background image is responsive, adapting its width to your visitor’s screen size.
Image Position. This panel allows you to adjust how your custom image background is positioned on your landing page. If you'd like an image to repeat, you'll need to select one of the Tile options.
Hide center column. By selecting this, you'll be able to hide or show the center white column.
Background color. Using this will allow you to select a custom color for your landing page.