$(document).ready(function() { 
    jQuery.fn.centerScreen = function(loaded) { 
        var obj = this; 
        if(!loaded) { 
            obj.css('top', $(window).height()/2- this.height()/2); 
            obj.css('left', $(window).width()/2- this.width()/2); 
            $(window).resize(function() { obj.centerScreen(!loaded); }); 
        } else { 
            obj.stop(); 
            obj.animate({ top: $(window).height()/2- this.height()/2, left: $(window).width()/2-this.width()/2}, 200, 'linear'); 
        }
    }
});