function roll(id){

   var objSrc = id.src;
   var ext = (objSrc.indexOf(".jpg") != -1) ? ".jpg" : ".gif";
   var idLength = objSrc.length;

   if(objSrc.indexOf("_over") != -1){
      var path = objSrc.substr(0, idLength - 9);
      id.src = path + ext;
   } else {
      var path = objSrc.substr(0, idLength - 4);
      id.src = path + "_over" + ext;
   }

}