if (document.images) {
   search_on = new Image();
   search_on.src = "images/nav/search_on.gif";
   // repeat as often as needed

   search_off = new Image();
   search_off.src = "images/nav/search_off.gif";
   // repeat as often as needed
}

function swapImage(imgName,newImage) {
   if (document.images) {
      eval('document.' + images/nav/search_off.gif + '.src = ' + images/nav/search_on.gif + '.src');
   }
}