mores

MetaBackGround will let you specify a post background by adding a custom field. This is not the sorcerer’s stone, but some users are not comfortable with php and editing of theme files, so all they need to do is install this plugin.

MetaBackGround

I once created this plugin for someone ... they needed to set backgrounds on a per-post basis.

wpxpertise

Lets you to set background images or a background videos on static pages. 
The background image/video can apply to whole page or just a specific section in that document. You can set as many background images/videos to a various sections of any given page.
Responsiveness is baked in to ensure the right media size is used at each layout breakpoint matching your active theme.
The background image/video can fill the entire screen if applied to the body tag, the wrapper or any HTML element of the current page that covers the viewport.
An overlay is applied to the background image/video to give enough contrast to the content over that background.
This plugin gives you more control and flexibility than the Gutenberg editor’s “Cover” and “Media & Text” blocks.
Install and try it then write a review to help us improve this plugin. We appreciate your feedback.
<h3>Settings</h3>
<ol>
<li>
From the left sidebar navigation of your WordPress control panel, look for the menu item labeled “WPXP BG Media” then click on it to open.
</li>
<li>
The information under the “Dashboard” tab of the opened page gives you enough details on how to define settings in the “Settings” and “Overlay” tabs.
</li>
<li>
The “Settings” tab is where you specify your active theme’s layout breakpoints. This allows the plugins to generate CSS media queries to match those breakpoints. Then for each breakpoint, a copy of the background image will be created with a width matching that layout breakpoint. All you have to do is upload to your WordPress media library an original image that is large enough for use at fullscreen on large displays (e.g. 4K).
</li>
<li>
In the “Overlay” tab, you define the background color and opacity of the “Background Media Overlay” and the “Content Overlay”.
</li>
</ol>

WPXP Background Media

Lets you to set background images or a background videos on static pages.

MetaBackGround vs WPXP Background Media