div.shadow-dialog { position:fixed;top:90px; left:0; width:100%; bottom:60%; z-index:2; }
div.shadow-dialog img { height:100%; position:absolute;top:0; left:0;width:100%;}
<div class="shadow-dialog"><img alt="" src="img/shadow.png"></div>
.gradient {
background: -moz-linear-gradient(top, transparent 0%, #ffffff 100%);
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%, transparent), color-stop(100%, #ffffff));
background: -webkit-linear-gradient(top, transparent 0%, #ffffff 100%);
background: -o-linear-gradient(top, transparent 0%, #ffffff 100%);
background: -ms-linear-gradient(top, transparent 0%, #ffffff 100%);
background: linear-gradient(top, transparent 0%, #ffffff 100%);
}