functions.php

add_action( 'genesis_entry_content', 'os_show_featured_image_single_posts', 9 );
function os_show_featured_image_single_posts() {
	global $post;


	if ( ! is_singular( 'post' ) ) {
		return;
	}

	// Get the featured image HTML
	$image_html_args = array(
		'size' => 'post-image',
		'attr' => array(
			'class' => 'aligncenter featured-image size-full wp-image-' . $post->ID .'',
		),
	);
	$image_html = genesis_get_image( $image_html_args );

	// Get the featured image URL for the lightbox a href only
	$image_url_args = array(
		'format' => 'url',
	);
	$image_url = genesis_get_image( $image_url_args );

	// Display the Image using both calls above
	echo '<a rel="lightbox" href="' . $image_url . '">'. $image_html .'</a>';
}