<script>

var banners = ['index4.jpg', 'index2.jpg','index1.jpg'];

var idx = 0;

setInterval(function(){

if(idx < banners.length){

document.querySelector('.container-lb').style.background = 'url(img/' + banners[idx] + ')';

}else{

idx = 0;

document.querySelector('.container-lb').style.background = 'url(img/' + banners[idx] + ')';

}

idx++;

}, 4000);

</script>