var imgs = $$('.fadein img'); 
var rand = Math.floor(Math.random()*imgs.length); 
for (var i = imgs.length - 1; i > rand; i--) { 
imgs[i].style.display = 'none'; 
}

var duration = 4000;
var showNextImage = true;
function stop_slideshow() {
showNextImage = false; 
} 
function start_slideshow() {
showNextImage = true; 
}
setInterval(function(){
if(!showNextImage){ return; }
var imgs = $$('.fadein img'), visible = imgs.findAll(function(img){ return img.visible(); });
if(visible.length > 1) {
visible.last().fade({ duration: 1 });
} else {
imgs.last().appear({ duration: 1, afterFinish: function(){ imgs.slice(0,imgs.length-1).invoke('show'); } });
}
}, duration);