Rewrite of http://davidchu.net/blog/2013/04/wordpress-child-pages-inherit-parent-pages-featured-image-genesis-style/

functions.php

<?php

add_action( 'genesis_before_post', 'dc_blah_feat' );
/**
 * Echo parent page post thumbnail for a page.
 */
function dc_blah_feat () {
	if ( is_page() )
    	echo prefix_get_page_parent_featured_image();
}

/**
 * Get featured image of current page's parent page.
 *
 * @return string Post thumbnail markup, or empty string if not currently on a page.
 */
function prefix_get_page_parent_featured_image() {
	if ( ! is_page() )
		return '';
	$parents = get_post_ancestors( get_the_ID() );
    $id = $parents ? $parents[ count( $parents ) - 1 ] : get_the_ID();
    return get_the_post_thumbnail( $id, 'full' );
}

This is a couple of extra lines of code longer than the original, but you get:

• Re-usable function that can find the parent page featured image for all themes, not just Genesis, and one that can be hooked in to multiple places, not just before post in Genesis.
• Code standards.
• Documentation.
• Avoidance of global $post.

Thanks again, Gary. Just used this in a slightly different way. Does the conditional check prevent the need for the global $post?