function imgfade() {
  var $alt = $('#imgbox IMG.toplevel');

  if($alt.next().length)
    var $neu = $alt.next();
  else
    var $neu = $('#imgbox IMG:first');

  $alt.addClass('middlelevel');
  $alt.removeClass('toplevel');
  $neu.css({opacity: 0.0});
  $neu.addClass('toplevel');

  $neu.animate({opacity: 1.0}, 2000, function() {
    $alt.removeClass('middlelevel');
  });
}