That will take some creative CSS. Basically you have to target that post or page from the body ID or article ID from the page (post) source code that is generated then you need to create a background CSS for the div container that you need the background image in. It’s more complicated than it sounds.
I would save it for a rainy day and play with a duplicate of your site somewhere to test things so that your site visitors don’t see things go upside down :silly: and sideways :side: