function newImage(arg) {
   if (document.images) {
     rslt = new Image();
     rslt.src = arg;
     return rslt;
   }
 }

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

 var preloadFlag = false;
 function preloadImages() {
   if (document.images) {
     nav_unternehmen_over = newImage("images/nav_unternehmen-over.gif");
     nav_leistung_over = newImage("images/nav_leistung-over.gif");
     nav_moeglichkeiten_over = newImage("images/nav_moeglichkeiten-over.gif");
     nav_kontakt_over = newImage("images/nav_kontakt-over.gif");
     preloadFlag = true;
   }
 }