Wordpress: saving default post thumbnail

gistfile1.txt

//Paste this in function.php
add_action( 'save_post', 'wptuts_save_thumbnail' );

function wptuts_save_thumbnail( $post_id ) {

	// Get Thumbnail
	$post_thumbnail = get_post_meta( $post_id, $key = '_thumbnail_id', $single = true );

	// Verify that post is not a revision
	if ( !wp_is_post_revision( $post_id ) ) {
		// Check if Thumbnail exists
		if ( empty( $post_thumbnail ) ) {
			// Add thumbnail to post
			update_post_meta( $post_id, $meta_key = '_thumbnail_id', $meta_value = 'ATTATCHMENT_ID' );
		}
	}

}

//in the loop
if (has_post_thumbnail()) {
	the_post_thumbnail();
}
else {
	echo '<img src="' . get_bloginfo('template_directory') . '/images/thumb-default.png' . '" width="100" height="100" alt="thumbnail" />';
}