Go to admin panel, Then Extensions → Templates. Select the template → select option tab → Then select layouts. By default Layouts will select default layout.

You need to select your layout override first because layout override depend page layout setting.

Select / change background image from here (For more information:  https://bdthemes.com/support/knowledge-base/understanding-about-layout-of-warp-framework/)

Then click save and close button for save your settings.