function rand ( n )
{

  	return ( Math.floor ( Math.random ( ) * n + 1 ) );

}

var images = new Array();

	images[1] = "images/main.jpg"
	images[2] = "images/main2.jpg"
	

function changeStyle()
{
var image = "url(" + images[rand(2)] + ")";
document.getElementById("mainbg").style.backgroundImage=image;

}