if (document.images) {

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

  serviceon = new Image();
  serviceon.src = "nav/service_on.gif";
  serviceoff = new Image();
  serviceoff.src = "nav/service_off.gif";

  resourceson = new Image();
  resourceson.src = "nav/resources_on.gif";
  resourcesoff = new Image();
  resourcesoff.src = "nav/resources_off.gif";

  clientson = new Image();
  clientson.src = "nav/clients_on.gif";
  clientsoff = new Image();
  clientsoff.src = "nav/clients_off.gif";

  abouton = new Image();
  abouton.src = "nav/about_on.gif";
  aboutoff = new Image();
  aboutoff.src = "nav/about_off.gif";

  contacton = new Image();
  contacton.src = "nav/contact_on.gif";
  contactoff = new Image();
  contactoff.src = "nav/contact_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');
  }
}

function turnOffAll() {
  if (document.images) {
    document.service.src = serviceoff.src;
  }
}