fotos = new Array()
images_count = 9;

for (var i = 0; i <= images_count; i++) {
  fotos[i] = "<img border=0 src=images/header_oben"+(i+1)+".jpg width=540 height=110>";
}

var nr = Math.round(images_count * Math.random());

var Image = fotos[nr];

document.write("<div align=left>" +Image+ "</div>");