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

preload('signs_up','images/nav-sub1.gif');
preload('signs_over','images/nav-sub1_over.gif');
preload('embroidery_up','images/nav-sub2.gif');
preload('embroidery_over','images/nav-sub2_over.gif');
preload('promoitems_up','images/nav-sub3.gif'');
preload('promoitems_over','images/nav-sub3_over.gif');
preload('awards_up','images/nav-sub4.gif');
preload('awards_over','images/nav-sub4_over.gif');
preload('cards_up','images/nav-sub5.gif');
preload('cards_over','images/nav-sub5_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");
}