jQuery.fn.centerOnScreen = function(loaded) {
    var obj = this;
    if(!loaded) {
        if ($(window).height() > this.height()) {
            obj.css('position', 'absolute');
            obj.css('top', $(window).height()/2-this.height()/2);
            obj.css('left', $(window).width()/2-this.width()/2);        
        } else {
            obj.css('position', 'absolute');
            obj.css('top', 0);
            obj.css('left', $(window).width()/2-this.width()/2);        

        }
        $(window).resize(function() {
            obj.centerOnScreen(!loaded); 
        });
    } else {
        obj.stop();
        if ($(window).height() > this.height()) {
            obj.css('position', 'absolute');
            obj.animate(
                { 
                    top: $(window).height()/2-this.height()/2, 
                    left: $(window).width()/2-this.width()/2
                }, 
                200, 
                'linear'
            );        
        } else {
            obj.css('position', 'absolute');
            obj.animate(
                {
                    top: 0,
                    left: $(window).width()/2-this.width()/2
                }, 
                200, 
                'linear'
            );        
        }

    }
}