function swap(id) {

   var url = document.getElementById(id).src;
   var off = url.replace(url.substring(url.length -6),'of.gif');
   var on = url.replace(url.substring(url.length -6),'on.gif');
   
   
   //alert('1 '+ url+'\n2 '+off+'\n3'+on+'\n replace'+url.charAt(url.length -5));
  
  
   if (url == off){
   
         document.getElementById(id).src = on; 
   
   } 
   else if (url == on) {
   
         document.getElementById(id).src = off; 
   }
}