Default/fallback post thumbnail image.

functions.php

<?php


/**
 *
 * Default/fallback post thumbnail image.
 *
 */
function aoa_filter_thumbnail_id( $thumbnail_id, $post = null ) {
	if ( ! $thumbnail_id ) {
			$thumbnail_id = 16157; //id of default featured image
		}

		return  $thumbnail_id;
} 
add_filter( 'post_thumbnail_id', 'aoa_filter_thumbnail_id', 20, 5 );