Kailey (trepmal)

The ability to select an image from the media library as the background is baked right into core now. This plugin will no longer be developed.
Add an action button to images in the media library. Easily use any image as your custom background.
Custom backgrounds must be enabled on the active theme

Get Background From Library

(No longer in development.) Add an action button to images in the media library. Easily use any image as your custom background

Travis Smith

By default, <a href="http://wpsmith.net/get-genesis" title="Genesis" rel="nofollow ugc">Genesis</a> doesn’t turn on WordPress Custom Backgrounds. Genesis Custom Backgrounds enables custom backgrounds and creates an option to enable selection of a default background from the sets of backgrounds provided by <a href="http://wpsmith.net/go/studiopress" title="StudioPress" rel="nofollow ugc">StudioPress</a>. Currently, it is broken down to Dark and Light backgrounds with subfolders. You can easily provide the user more options by simply uploading more backgrounds with in either the light or dark folders. Upload using the following structure, folder: camo & files: camo-1.png, camo-2.png, and camo-3.png (in all lowercase). Furthermore, you can also upload a custom background via the WordPress Custom Background via Appearance > Background. You can easily add customization ability for the site to easily switch default backgrounds or upload their own (which will always overwrite the default).
IMPORTANT: 
You must have <a href="http://wpsmith.net/get-genesis" title="Learn more about Genesis" rel="nofollow ugc">Genesis</a> installed. Click <a href="http://wpsmith.net/get-genesis" title="Learn more about Genesis" rel="nofollow ugc">here</a> to learn more about <a href="http://wpsmith.net/get-genesis" title="Learn more about Genesis" rel="nofollow ugc">Genesis</a>
<h3>Special Thanks</h3>
I owe a huge debt of gratitude to all the folks at <a href="http://wpsmith.net/get-genesis/" title="StudioPress" rel="nofollow ugc">StudioPress</a>, their <a href="http://wpsmith.net/get-genesis/" title="StudioPress Themes" rel="nofollow ugc">themes</a> make life easier.

Genesis Custom Backgrounds

This plugin provides the option to select a default custom background.

Get Background From Library vs Genesis Custom Backgrounds