if (document.images) {

  differenton = new Image();
  differenton.src = "nav/different_on.gif";
  differentoff = new Image();
  differentoff.src = "nav/different_off.gif";

  portfolioon = new Image();
  portfolioon.src = "nav/portfolio_on.gif";
  portfoliooff = new Image();
  portfoliooff.src = "nav/portfolio_off.gif";

  contacton = new Image();
  contacton.src = "nav/contact_on.gif";
  contactoff = new Image();
  contactoff.src = "nav/contact_off.gif";

  homeon = new Image();
  homeon.src = "nav/home_on.gif";
  homeoff = new Image();
  homeoff.src = "nav/home_off.gif";
}

function turnOn(imageName) {
  if (document.images) {
    document[imageName].src =
      eval(imageName + 'on.src');
  }
}

function turnOff(imageName) {
  if (document.images) {
    document[imageName].src =
      eval(imageName + 'off.src');
  }
}