Phillip.Gooch

Backdrop is a site background customizer with power greater than the mere stock background selector included with WordPress. What can backdrop do that the default can’t? I’m glad you asked…
<ul>
<li>More than just boring backgrounds, flexible backgrounds, parallax backgrounds, even sliding backgrounds are stuffed in here too (don’t worry, the boring ones are still around as well).</li>
<li>Options, options, options! More options that you can shake a well sharpened stick at.</li>
<li>Uses the WordPress customizer interface so you can see your changes as you make them.</li>
<li>One click HiDpi (or retina) backgrounds, make it look all purdy on that fancy screen.</li>
<li>Theme schmeme, we don’t need support in your stinkin theme, backdrop can force it’s way into nearly any theme.</li>
<li>Calls external JS and CSS files that are appropriately cached.</li>
<li>Advanced options for those in the know with CSS.</li>
<li>Built for the smoothest animations possible.</li>
</ul>
Note: Starting with version 2.0 Backdrop will no longer work it’s magic with older versions of Internet Explorer (before version 10). If you want to support animations on older versions of IE then please use Backdrop 1.X.
Note: Your Backdrop settings will not be transfered from 1.X to 2.X, you will need to re-set your backdrop on update.

Backdrop

Backdrop is an improved site background customizer allowing for all manner of fancy things.

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.

Backdrop vs Simple Full Screen Background Image