http://sridharkatakam.com/add-responsive-image-site-header-genesis/

gistfile1.txt

//* Add support for structural wraps
add_theme_support( 'genesis-structural-wraps', array(
	'header',
	'nav',
	'subnav',
	'site-inner',
	'footer-widgets',
	'footer',
	'header-image'
) );

add_action ( 'genesis_before_header', 'sk_add_header_image' );
function sk_add_header_image() {
	printf( '<div %s>', genesis_attr( 'header-image' ) );
	genesis_structural_wrap( 'header-image' );

		$header_image = sprintf( '<a href="%s" title="%s"><img src="'. get_stylesheet_directory_uri() .'/images/header.jpg" title="%s" alt="%s"/></a>', trailingslashit( home_url() ), esc_attr( get_bloginfo( 'name' ) ), esc_attr( get_bloginfo( 'name' ) ), esc_attr( get_bloginfo( 'name' ) ) );
		echo $header_image;

	genesis_structural_wrap( 'header-image', 'close' );
	echo '</div>';
}

style.css

.header-image {
	margin-top: 4rem;
	margin-bottom: 4rem;
	text-align: center;
}

.header-image img {
	margin-bottom: 0;
	vertical-align: top;
}

@media only screen and (max-width: 320px) {
	.header-image {
		margin-top: 2rem;
		margin-bottom: 2rem;
	}

}