Use filter “og_image_init”:

add_filter('og_og_image_value', 'my_og_og_image_value');
function my_og_og_image_value($images)
{
    if ( empty($images) ) {
        $images[] = 'http://wordpress/wp-content/uploads/2014/11/DSCN0570.jpg';
    }
    return $images;
}