setInterval(function(){
  $$('.fadein').each(function(fadein){
    var imgs = fadein.childElements();
    var visible = imgs.findAll(function(img){ return img.visible(); });
    console.log(imgs);
    
    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');
        }
      });
    }
  })
}, 6000);