This simple question could easily open a whole can of worms.
Really depends on what skin you are using, and what you want the background image to do (repeat, stretch, center, etc.).
Lets say you are using ideal_comfort, and you want a repeating background image.
Code:
html, body{
background: #4fd6f0 url(http://www.freeimagehosting.net/newuploads/lgqjd.jpg) center top;
}
This code also sets the background color.
I made you a quick playground where you can experiment:
http://jsfiddle.net/JE3E8/1/