var clsname = new Array();
clsname[0] = "logobg1";
clsname[1] = "logobg2";
clsname[2] = "logobg3";
clsname[2] = "logobg4";
clsname[3] = "logobg5";

function rnd() {
  var r = Math.random();
  var idx = Math.floor(r*clsname.length);
  return clsname[idx];
}

function chbg() {
  document.getElementById('pgheader').className=rnd();
  return 0;
}