var imagenumber = 10 ; //Anzahl der Grafiken
var randomnumber = Math.random();
var rand1 = Math.round( (imagenumber-1) * randomnumber) + 1;

images = new Array
images[1] = "/fileadmin/img/header/header1.jpg";
images[2] = "/fileadmin/img/header/header2.jpg";
images[3] = "/fileadmin/img/header/header3.jpg";
images[4] = "/fileadmin/img/header/header4.jpg";
images[5] = "/fileadmin/img/header/header5.jpg";
images[6] = "/fileadmin/img/header/header6.jpg";
images[7] = "/fileadmin/img/header/header7.jpg";
images[8] = "/fileadmin/img/header/header8.jpg";
images[9] = "/fileadmin/img/header/header9.jpg";
images[10] = "/fileadmin/img/header/header10.jpg";

var image = images[rand1] 

//document.getElementById("main_menu_container").style.backgroundImage = image;