function zufall(){
aBilder = new Array ("bg/sf_bg01.jpg", "bg/sf_bg02.jpg", "bg/sf_bg03.jpg", "bg/sf_bg04.jpg", "bg/sf_bg05.jpg", "bg/sf_bg06.jpg")
var index = Math.floor(Math.random()*aBilder.length);
document.getElementById("content").style.backgroundImage = "url("+aBilder[index]+")";
}