// <a href="html-or.jpg" onclick="return popup(this,123,456)" title="..."
// or
// <a href="html-or.jpg" onclick="return popup(this)" title="..."

var pop = null;

function popdown() {
  if (pop && !pop.closed) pop.close();
}

function popup(obj,w,h,c,left,top) {
  var url = (obj.getAttribute) ? obj.getAttribute('href') : obj.href;
  if (!url) return true;
  var left = (c==1) ? (screen.width/2)-(w/2) : left;
  var top  = (c==1) ? (screen.height/2)-(h/2) : top;
  var args = 'left=' + left + ',top=' + top + ',width='+w+',height='+h+',resizable,scrollbars';
  popdown();
  pop = window.open(url,'',args);
  return (pop) ? false : true;
}

//window.onunload = popdown;