here is way to define you're bacground to set it like in blog where you move only main window not the background
it helps if you have cool background but it's too small for page and makes cracks where it repeats
looks realy nice try it !! (maybe easy code but maybe will help someone i was looking for this for a time so don't say it's useles )
put it in root>themes>you're theme name > style.css
replace orginal body with this or edit a bit
PHP Code:
body {
font-family : sans-serif;
background : url(pics/youre background name goes here.png);
background-attachment: fixed;
background-repeat: no-repeat;
background-position: top left;
font-size : 15px;
}