I have done this already. It’s changed forum posts, but not my main URL as above.

function my_default_og_image( $image_url, $type ){
    if( $type == 'og:image' && !$image_url){
        return 'https://example.com/images/mylogo.jpg';
    }
    else{
        return $image_url;
    }
}

add_filter('wpforo_find_image_url', 'my_default_og_image', 10, 2);