kukuhnovaputra on "How to make responsive header image on twenty twelve theme?"

I want to make responsive header image on my twenty twelve theme, I've input CSS code in CSS Stylesheet Editor.

#header_area {
	background: url('http://kukuhnovaputra.com/wp-content/uploads/2013/03/Header.png');
	background-repeat: no-repeat;
	background-size: 100%;
}

But, when i try to resize my browser window. This code is not work.
My blog is kukuhnovaputra.com