Display featured images on select single posts in Genesis.

display_featured_image.php

<?php
add_action( 'genesis_before_entry', 'featured_post_image', 8 );
/**
 * Add the post thumbnail to single post
 */
function featured_post_image() {
	if ( ! is_singular( 'post' ) )
		return;
	
	the_post_thumbnail( 'post-image' );
}