.mainview {
    background-image: url(images/bg-1.png);
    height: 600px;
    background-repeat: no-repeat;
    background-size: cover;
    background-position: bottom center;
    background-attachment: fixed;
    position: relative;
    top: 12px
}

<script type="text/javascript">

  var images = ['image-1.jpg', 'image-2.jpg', 'image-3.jpg', 'image-4.jpg'];
  document.getElementsByClassName('mainview')[0].style.backgroundImage = 'url(' + images[Math.floor(Math.random() * images.length)] + ')';

</script>