function preload(imgObj,imgSrc){
  if(document.images) {
    eval(imgObj+' = new Image()');
    eval(imgObj+'.src = "'+imgSrc+'"');
  }
}

preload('home_up','images/nav-home-up.gif');
preload('home_over','images/nav-home-over.gif');
preload('bio_up','images/nav-bio-up.gif');
preload('bio_over','images/nav-bio-over.gif');
preload('rates_up','images/nav-rates-up.gif');
preload('rates_over','images/nav-rates-over.gif');
preload('references_up','images/nav-references-up.gif');
preload('references_over','images/nav-references-over.gif');
preload('services_up','images/nav-services-up.gif');
preload('services_over','images/nav-services-over.gif');
preload('contact_up','images/nav-contact-up.gif');
preload('contact_over','images/nav-contact-over.gif');


function changeImage(layer,imgName,imgObj){
    if(document.layers && layer!=null) eval('document.'+layer+'.document.images["'+imgName+'"].src = '+imgObj+'.src');
    else document.images[imgName].src = eval(imgObj+".src");
}