setInterval(function(){
  var imgs = $$('.slideshow a img'),
   visible = imgs.findAll(function(img){ return img.visible(); });
  if(visible.length>1) visible.last().fade({ duration: .3 });
    else imgs.last().appear({ duration: .3,
      afterFinish: function(){ imgs.slice(0,imgs.length-1).invoke('show');  } });
}, 15000);