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.

shazdeh

This plugin enables you to display your RokGallery galleries as an slideshow in the background of your site. Requires the <a href="http://www.rockettheme.com/" rel="nofollow ugc">RokGallery</a> plugin for WordPress.

RokGallery Background Slideshow

Display your RokGallery galleries as an slideshow in the background of your site.

Simple Slideshow Background vs RokGallery Background Slideshow