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.

AMP-MODE

Scott DeLuzio

Simple Full Screen Background Image will allow you to easily upload and set a full screen image as the background of your website. Images will be automatically scaled with the browser, so regardless of the browser size, the image will always fill the screen, even as the browser is resized live.
Once activated, go to Appearance > Fullscreen BG Image. From here click Choose Image, then either upload from your computer or choose one from the media library. Once you have chosen your image, select the size you wish to insert and click Insert Into Post. A preview of your image will appear below. Now click Save Options 
and view your site. The image should now be applied as a full screen background image.
<h4>Go Pro!</h4>
A greatly enhanced Pro version is <a href="https://fullscreenbackgroundimages.com/downloads/full-screen-background-images-pro/" rel="nofollow ugc">available</a>! Features of the pro version include:
<ul>
<li>Unlimited background images</li>
<li>Post / page-specific background images</li>
<li>Multiple images with fade transitions on pages</li>
</ul>
Learn more about the Pro version <a href="https://fullscreenbackgroundimages.com/downloads/full-screen-background-images-pro/" rel="nofollow ugc">here</a>.

Simple Full Screen Background Image

This plugin provides a simple way to set an automatically scaled full screen background image.

MetaBackGround vs Simple Full Screen Background Image