gistfile1.html

 <style>
#box1 {

   position:absolute;
             
                top:0;
                left:0;
                width:100%;
                height:100%;
                z-index:-1;
                opacity: 0;
                background: transparent url(img/back1.jpg); 
                background-repeat: no-repeat center center fixed;
                -webkit-background-size: cover;
                -moz-background-size: cover;
                -o-background-size: cover;
                background-size: cover; 
                display: none;



}
 
</style>

<div id="box1" class="box"></div>

<script type="text/javascript">
var i = 0, boxtmp;

if (i === 0) {
  boxtmp='box1';
  $("#"+boxtmp).animate( {opacity: 1, display: "inline"}, 2000 );
  i = 1;
}

</script>