How can I give my posts a background image?

Hi there,

Just to confirm, you are trying to add a background image as in Customizer > Background images > Content but only for single posts?

Let me know :)

I have no such thing as:
Customizer > Background images > Content

Iv'e seen some documentation and question in forum, My menu is different.
See my menu with translate.

https://ibb.co/ZSkyH7C
There is Additional CSS in the end. Where are the Background Images?
And yes, I want to set is to posts only (All posts).

Is the background module activated?
https://docs.generatepress.com/article/installing-gp-premium/#activate-your-modules

If you just want it for the single posts then we should just use CSS:

body.single .inside-article {
    background-image: url(http://IMAGE-URL-HERE);
}

Adding CSS: https://docs.generatepress.com/article/adding-css/