Make featured image first in WP Gallery, no matter what order it's set in.

featured-image-1.php

<?php

add_filter( 'shortcode_atts_gallery', 'test_post_gallery' );
function test_post_gallery( $args ) {

  $feat_id = get_post_thumbnail_id();
	// bail if no feat image ID
	if ( !$feat_id )
		return $args;

	$feat_id = $feat_id.',';
	// is our feat imaage ID in in the gallery?
	if ( strpos( $args['include'], $feat_id ) !== false ) {
		// if so, move it to the front
		$args['include'] = $feat_id . str_replace( $feat_id, '', $args['include'] );
	}
	return $args;
}