if (document.images) {
  var home = new Image(); // for the inactive image
  home.src = "images/home.gif";
  var homeover = new Image(); // for the active image
  homeover.src = "images/home_over.gif";
}

function fnhome() {
  if (document.images)
    document.images.home.src = home.src;
}

function fnhomeover() {
  if (document.images)
    document.images.home.src = homeover.src;
}