$(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');
        }
    }

});