You are not saying where you want the background image…
In the overall background (blue area), or in the content? If in the content is t just the are below the menu bar?
Without the “Plus” plugin, the way to add a background image is to add the CSS (like below) in the area BG CSS+ box
{background:url(ImageUrl) no-repeat;}
The options needed inside that rule like cover or contain, positioning etc,… will depend on what you want to achieve . Once the image is in place in the right area we can refine based on what you want