This plugin allows you to add and image as background of posts, pages or custom post type in general (in the same way as a thumbnail).
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<li>This is a rewrite of my old plugin <a href="https://wordpress.org/plugins/post-background/" rel="ugc">Post Background</a> which I do not have access anymore. *</li>
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WP Post Background

This plugin allows you to add and image as background of posts, pages or custom post type in general (in the same way as a thumbnail).

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.

WP Post Background vs Super Simple jQuery Parallax Background