[Serenity]

in the admincp>style manager>all style options(for the skin you want to change>body

use: #000000 url(path/to/image.gif)
change the hex color value to anything you want to use

u can set it to a static background so it doesn't move by using this code:
background-attachment: fixed;
in the Extra CSS Attributes box

as for having 2 different backgrounds i think you would have to make a child style for people to switch to.