var message="Our system has prevented you from downloading this image.  Please contact webmistress for assistance.";

function click(e){
   if (document.all) {
      if (event.button==2||event.button==3) {
         alert(message);
         return false;
      }
   }
   if (document.layers) {
      if (e.which == 3) {
         alert(message);
         return false;
      }
   }
}
if (document.layers) {
   document.captureEvents(Event.MOUSEDOWN);
}
document.onmousedown=click;