function changeImages() {
  if (document.images && (preloadFlag == true)) {
    for (var i=0; i<changeImages.arguments.length; i+=2) {
      document[changeImages.arguments[i]].src = "/images/" + changeImages.arguments[i+1];
    }
  }
}

var preloadFlag = false;
function preloadImages() {
  if (document.images) {
    nav_svco = new Image(); nav_svco.src="/images/nav_svco.gif";
    nav_prco = new Image(); nav_prco.src="/images/nav_prco.gif";
    nav_veho = new Image(); nav_veho.src="/images/nav_veho.gif";
    nav_loco = new Image(); nav_loco.src="/images/nav_loco.gif";
    nav_cono = new Image(); nav_cono.src="/images/nav_cono.gif";
    nav_homo = new Image(); nav_homo.src="/images/nav_homo.gif";
    preloadFlag = true;
  }
}