function changeBackgroundImage() {
	var aBackgroundImages = new Array("habst_enso_SL_front_2.jpg" , "habst_enso_SL_front_3.jpg");
	var nArrayLength = aBackgroundImages.length;
	var oRandomImageNummber = Math.floor( Math.random() * nArrayLength);
	document.getElementById('contentId').style.backgroundImage = 'url(wallpaper/' + aBackgroundImages[oRandomImageNummber] + ')' ;
}