How to stop right click for Guests not for the register members

[nomoreturn]

Hello every one i want to use this code to avoid right click in post

<script language=JavaScript> <!--
 var message="Why not Register first ? You are not allowed to save images .";
 function click(z) {
  if (document.all) {
   if (event.button == 2) {
    alert(message);
    return false;
   }
  }
  if (document.layers) {
   if (z.which == 3) {
    alert(message);
    return false;
   }
  }
 }
 if (document.layers) {
  document.captureEvents(Event.MOUSEDOWN);
 }
 document.onmousedown=click;
// --> </script>

but its working for both register member and Guests how i can change it just for geusts not for the Register members so register members can right click and copy my stuf please help me to do this thanks