How To Get Popup Window Centered

I am implementing a pop up window and need it to be centered. Found this code (below) seems to work good. I need to open it with an image rather than text link.

<script type="text/javascript">
<!--
function popup(url){
var width = 300;
var height = 200;
var left = (screen.width - width)/2;
var top = (screen.height - height)/2;
var params = 'width='+width+', height='+height;
params + = ', top='+top+', left='+left;
params + = ', directories=no';
params + = ', location=no';
params + = ', menubar=no';
params + = ', resizable=no';
params + = ', scrollbars=no';
params + = ', status=no';
params + = ', toolbar=no';
newwin=window.open(url,'windowname5', params);
if (window.focus) {newwin.focus()}
return false;
} // -->
</script>
<a href="javascript: void(0)"onclick="popup('popup.html')">Centered popup window</a>