скролим кликаем по картинке и она всегда в центре ...
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8" />
<script src="http://code.jquery.com/jquery-latest.js"></script>
<style type="text/css">
body{
height: 1000px;
}
</style>
<script>
jQuery.fn.center = function()
{
var w = $(window);
this.css("position","absolute");
this.css("top",(w.height()-this.height())/2+w.scrollTop() + "px");
this.css("left",(w.width()-this.width())/2+w.scrollLeft() + "px");
return this;
}
function creatOverlay(){
$(this).center()
}
$(document).on({"click": creatOverlay}, "img")
</script>
</head>
<body>
<img src="http://javascript.ru/forum/images/ca_serenity/misc/logo.gif" alt="" >
</body>
</html>