Create Default Fallback Image For Jetpack Related Posts

functions.php

function jp_custom_image( $media, $post_id, $args ) {
    if ( $media ) {
        return $media;
    } else {
        $permalink = get_permalink( $post_id );
        $url = apply_filters( 'jetpack_photon_url', 'https://example.com/wp-content/uploads/2017/07/example-image.jpg' );
      
        return array( array(
            'type'  => 'image',
            'from'  => 'custom_fallback',
            'src'   => esc_url( $url ),
            'href'  => $permalink,
        ) );
    }
}
add_filter( 'jetpack_images_get_images', 'jp_custom_image', 10, 3 );