Add a Default Image to Jetpack Related Posts

functions.php

<?php
function jrp_custom_image( $media, $post_id, $args ) {
    if ( $media ) {
        return $media;
    } else {
        $permalink = get_permalink( $post_id );
        $url = apply_filters( 'jetpack_photon_url', '[image url]' );

        return array( array(
            'type'  => 'image',
            'from'  => 'custom_fallback',
            'src'   => esc_url( $url ),
            'href'  => $permalink,
        ) );
    }
}
add_filter( 'jetpack_images_get_images', 'jrp_custom_image', 10, 3 );
?>

Due to some changes to my site a number of posts no longer have featured images and may not include an image either. This code allows you to specify an image that will appear as a default in such situations.

Add to your theme's functions.php and replace [image url] with the full URL to the default image.