[Jetpack] Add a default fallback image if no image can be found in a post

plugin.php

<?php
/*
 * Plugin Name: Jetpack default image
 * Plugin URI: http://wordpress.org/extend/plugins/
 * Description: Add a default fallback image if no image can be found in a post
 * Author: Jeremy Herve
 * Version: 1.0
 * Author URI: http://jeremyherve.com
 * License: GPL2+
 */

function jeherve_custom_image( $media, $post_id, $args ) {
	if ( empty( $media ) ) {
		$permalink = get_permalink( $post_id );
		$url = apply_filters( 'jetpack_photon_url', 'YOUR_LOGO_IMG_URL' );
	
		return array( array(
			'type'	=> 'image',
			'from'	=> 'custom_fallback',
			'src'	=> esc_url( $url ),
			'href'	=> $permalink,
		) );
	}
}
add_filter( 'jetpack_images_get_images', 'jeherve_custom_image', 10, 3 );