setInterval(function(){
  var imgs = $$('.slideshow img.active'), 
   visible = imgs.findAll(function(img){ return img.visible(); });
  if(visible.length>1) visible.last().fade({ duration: .8 });
  else 
  imgs.last().appear({ duration: .8, afterFinish: function(){ imgs.slice(0,Math.floor(Math.random() * imgs.length)).invoke('show');  } });
}, 3000);