function rand(){
    return Math.round(Math.random()*(css.length-1));
}

var css = new Array(
	'<style type="text/css">div#header {background-image: url(/media/images/header_bg.jpg);}</style>',
	'<style type="text/css">div#header {background-image: url(/media/images/header_bg1.jpg);}</style>',
	'<style type="text/css">div#header {background-image: url(/media/images/header_bg2.jpg);}</style>',
	'<style type="text/css">div#header {background-image: url(/media/images/header_bg3.jpg);}</style>',
	'<style type="text/css">div#header {background-image: url(/media/images/header_bg4.jpg);}</style>'
);

rand = rand();

document.write(css[rand]);