function showImage(imageName, windowWidth, windowHeight)
{
  var top = (screen.availHeight-windowHeight)/2;
  var left = (screen.availWidth-windowWidth)/2;
  var viewerPage = "viewer.php?image=" + imageName;
  var windowName = "screenshot";
  var parameters = "toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=no,resizable=no,width="+windowWidth+",height="+windowHeight+",top="+top+",left="+left;
  window.open(viewerPage, windowName, parameters);
}