barteled

Slideshow background adds a full size, smooth transition slideshow to the background of a WordPress site. Images can be selected from the media library or uploaded via drag-and-drop. Every page load will begin at a random position in the image loop so that users are not presented with the same start image every page change.
Additionally, the images maintain their aspects ratios during resize and turn off for mobile users.
This plug-in was conjured up by a nerd at <a href="http://think-bowl.com/" title="It's less of a tank and more of a bowl." rel="nofollow ugc">Think Bowl</a>

Simple Slideshow Background

Display a full size slideshow in the background featuring images from the media library.

Ryan Novotny

Add the particles.js JavaScript library to add cool particle effects to any WordPress page ( demo: <a href="https://particlebackground.com/" rel="nofollow ugc">https://particlebackground.com/</a> ).
Uses shortcode or easy options for adding to the front page or blog page of WordPress.
Fully customize the particles using your own JSON or the built in color & density settings
Leave a comment in the support forum for any suggestions or bugs.
<h4>Features</h4>
Add particle background to home or blog pages with one click 
Add particle background to any page using shortcodes 
Customize the background color and dot color 
Add any HTML just like a WordPress post to display on top of the particle background (optional)

Particle Background WP

Particle Backgrounds WP lets you easily add cool particle-style backgrounds to any page.

Simple Slideshow Background vs Particle Background WP