<script type="text/javascript">
var image = new Array ();
image[0] = "lib/images/1.jpg";
image[1] = "lib/images/2.jpg";
image[2] = "lib/images/3.jpg";
image[3] = "lib/images/4.jpg";
var size = image.length
var x = Math.floor(size*Math.random())
$('#random_img').attr('src',image[x]);
</script>

<img id='random_img'/>