gistfile1.html

 <style>
#box1 {

   position:absolute;
             
                top:0;
                left:0;
                width:100%;
                height:100%;
                z-index:-1;
                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>
 <script type="text/javascript">
 var i=0;

 
var boxtmp;

if(i=='0')
    {
         
        boxtmp='box1';
        $("#"+boxtmp).animate( {opacity: "1", display: "in-line"}, 2000 );
        i=1;
   
}
</script>


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