function highlightButton(id, event) {
   var src = document.getElementById(id).src;
   var basename = src.substring(0,src.lastIndexOf('_'));
   var extension = src.substring(src.lastIndexOf('.'));
   if(event == 'over') {
      document.getElementById(id).src = basename + '_active' + extension;
   }
   if(event == 'out') {
      document.getElementById(id).src = basename + '_norm' + extension;
   }
}