onload = function() {

images = new Array(
"images/title_startseite_komplett.jpg",
"images/title_startseite_baenke.jpg"

);

rand = Math.floor(Math.random()*images.length); // chose a random number, between 0 and the length of the array -1
currentImage = images[rand]; // set img to the random image's src

document.getElementById("randomimage").style.backgroundImage = "url('"+currentImage+"')"; //for div background
}