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.

tommygeorge

Super Simple jQuery Parallax Background enables a parallax effect with your theme’s background image. Just set a background image in your theme’s “customize” options (optionally setting it to “fixed” position) and activate this plugin. Done. “It just works.”
It relies on jQuery, which comes with WordPress and is probably built into your theme. Tested and works with the “Twenty Twelve” theme out of the box.
Looks best with seamlessly-tiling background images.

Super Simple jQuery Parallax Background

For themes with background images, creates a parallax effect when scrolling. Super simple; no configuration. Just works.

Simple Slideshow Background vs Super Simple jQuery Parallax Background