redwerks

This plugin allows posts and pages to have individual custom backgrounds. A full-body background that shows up on the post page itself can be specified. As well as background that just covers the content area and shows up on all places the post is visible.

Post Backgrounds

Allows posts and pages to have individual custom backgrounds. Both as body backgrounds and as content backgrounds.

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.

Post Backgrounds vs Super Simple jQuery Parallax Background