add_action ( 'genesis_before_header', 'sk_add_banner_image' );
function sk_add_banner_image() {

    if ( has_post_thumbnail() ) {

        echo '<div class="banner-image">
                <div class="wrap">
                    <img src="' . genesis_get_image( array( 'format' => 'url' ) ) . '" />
                </div>
            </div>';
    }

}