function resRoll(rollo) {
 var Rollo = "";
 var par_src = new Object;
 ( document.getElementById ) && ( Rollo = document.getElementById(rollo) );
 if (Rollo) {
  was_src = Rollo.src;
  par_src[0] = was_src.substring(0, was_src.lastIndexOf('.'));
  par_src[1] = was_src.substring(was_src.lastIndexOf('.'), was_src.length);
  sub_src = par_src[0].substring(0, par_src[0].length - 1);
  was_tail = par_src[0].charAt(par_src[0].length -1);
  new_tail = (was_tail == "A") ? "B" : "A";
  eval("Rollo.src = \""+sub_src+new_tail+par_src[1]+"\";");
 }
}