var imagenumber = 3; 
var randomnumber = Math.random();
var rand1 = Math.round( (imagenumber-1) * randomnumber) + 1;
sources = new Array;
sources[1] = "werbung1.gif";
sources[2] = "werbung2.gif";
sources[3] = "werbung3.gif";
var image = sources[rand1];