var maxnum = 3;
var num = 1;
var dirdefec = 'http://img.youtube.com/vi/';
var rotacion = 0;
var rotation;

function startThumbChange(pid,pdir){
  rotation = setInterval("changeThumb('" + pid + "')", 600);
}

function endThumbChange(pid,pdir){
  if(pdir == undefined) pdir = dirdefec;
  num = 1;
  document.images[pid].src = pdir + pid + "/" + num + '.jpg';
  clearInterval(rotation);
}

function changeThumb (pid,pdir){
  num++;
  if(num > maxnum) num = 1;
  if(pdir == undefined) pdir = dirdefec;
  document.images[pid].src = pdir + pid + "/" + num + '.jpg';
}