functions.php

<?php

/**
 * Default to most recent image 
 *
 * @author Bill Erickson
 * @link http://www.billerickson.net/code/default-to-most-recent-image
 *
 * @param array $defaults
 * @param array $args
 * @return array $defaults
 */
function ea_default_to_most_recent_image( $defaults, $args ) {

	$image_ids = array_keys(
		get_children(
			array(
				'post_parent'    => isset( $args['post_id'] ) ? $args['post_id'] : get_the_ID(),
				'post_type'	     => 'attachment',
				'post_mime_type' => 'image',
				'orderby'        => 'menu_order',
				'order'	         => 'ASC',
			)
		)
	);
	
	$defaults['num'] = key( array_slice( $image_ids, -1, 1, TRUE ) );

	return $defaults;
}
add_filter( 'genesis_get_image_default_args', 'ea_default_to_most_recent_image', 10, 2 );