facebook share with cutom image and popup window

<a href="#" onclick="Popup=window.open('https://www.facebook.com/sharer/sharer.php?u=http://silvergray.in/','Popup','toolbar=yes,location=no,status=no,menubar=no,scrollbars=no,resizable=no,width=420,height=400,left=430,top=23'); return false;">
</a>

CSS:

.fb a {
display:block;
width:2em;
height:2em;
float:left;
z-index:999;
position:fixed;
cursor:pointer;
z-index: 1002;
background:url(../images/icons/socialN.png) 0px -2px #fff;
white-space:nowrap;
cursor: pointer;
filter:progid:DXImageTransform.Microsoft.Alpha(opacity=70);
right: 10%;
top: 2%;
box-shadow: #000 0px 0px 5px;
}