var undefined;

var pics = [];
for (var i=0; i<7; i++){
pics[i] = new Image;
pics[i].src = 'http://www.rdts.ag/praesentation/mentorag/website/bilder/home/'+ (i+1) +'.jpg';
}

function getRandomNumber(n){
var bildnummer = 0;
if (n){
  var wert = n * Math.random();
  bildnummer = Math.floor(wert);
}
return bildnummer;
}

function getLayoutPic(){
var index = getRandomNumber(pics.length-1);
if (pics[index]){
  document.images['layoutpic'].src = pics[index].src;
} else {
  getLayoutPic();
}
}