From http://wordpress.org/support/topic/help-can-i-specify-a-defaultfallback-image?replies=1 Add a default fallback image to jetpack open graphif no image can be found in a post

jetpack_fallback_image.php

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 );