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// Add image sizes.
add_image_size( 'header', 1600, 400, TRUE );

// Add featured image section.
add_action( 'genesis_after_header', 'ambre_featured_image' );
function ambre_featured_image() {
	if ( is_singular( array( 'post', 'page' ) ) && has_post_thumbnail() ){
		echo '<div id="full-image">';
		echo get_the_post_thumbnail($thumbnail->ID, 'header');
		echo '</div>';
	}
}