Header image full width of page

on the twenty twelve theme I would like to make the header image with no white margins on the sides. The header image uploaed is 1024px but still shows at 960. Change code in header.php to the below, but still shows 960

<?php $header_image = get_header_image();
		if ( ! empty( $header_image ) ) : ?>
			<a>"><img src="<?php echo esc_url( $header_image ); ?>" class="header-image" width="100%" height="100%" alt="" /></a>

you can see it here