Backdrop is a site background customizer with power greater than the mere stock background selector included with wordpress. What can backdrop do that the default can't? I'm glad you asked...
- More than just boring backgrounds, flexible backgrounds and parallex backgrounds are stuffed in here too (don't worry, the boring ones are still around as well).
- Options, options, options! More options that you can shake a well sharpened stick at.
- Who said background had to scroll up when you scroll down, make it left right, or even up and to the right if you want.
- One click retina backgrounds, make it look all purdy on that fancy screen.
- Theme schmeme, we don't need support in your stinkin theme, backdrop can (and sometimes has to) force it's way into nearly any theme.
- Built with efficiency in mind, adds only what it needs to work, and uses cachable styles and script automagically generated by the plugin.
Note: Backdrop totally ditches those stock customizable backgrounds to work it's kung-fu, while it works most of the time it may not work for you, if this is the case let me know and I'll see if I get jimmy it in there for you.