function switch_imgs() {
   imgs = document.getElementsByTagName('img');
   newsrc = Array();
   newsrc_nid = Array();
   oldsrc = Array();
   for(i=0; i < imgs.length; i++) {
      estado = imgs[i].src.substr((imgs[i].src.length-7), 3);
      if(estado=='off') {
         oldsrc[imgs[i].id] = imgs[i].src;
         newsrc[imgs[i].id] = imgs[i].src.replace('_off.', '_on.');
         newsrc_nid[i] = imgs[i].src.replace('_off.', '_on.');
         imgs[i].onmouseover = function() {
            this.src = newsrc[this.id];  
         }
         imgs[i].onmouseout = function() {
            this.src = oldsrc[this.id];  
         }
      }
   }
   return newsrc_nid;
}