Решил вопрос сам, если кто-то столкнется - снизу решение, которое скачает все фото из последних пяти постов, каждого вашего диалога:

$dialogs = $MadelineProto->get_dialogs();
    $channels = [];
    foreach ($dialogs as $a => $b) {
        if (isset($b['channel_id'])) {
            $channels[] = "-100{$b['channel_id']}";
        }
    }
    $settings = [
        "peer" => "",
        "offset_id" => "0",
        "offset_date" => "0",
        "add_offset" => "0",
        "limit" => "5",
        "max_id" => "0",
        "min_id" => "0",
        "hash" => "0",
    ];
    $messages = [];
    foreach ($channels as $a) {
        $settings['peer'] = $a;
        $messages[] = $MadelineProto->messages->getHistory($settings);
    }
    foreach ($messages as $m) {
        foreach ($m['messages'] as $n) {
            if (isset($n['media']['photo'])) {
                $MessageMedia = $n['media']['photo'];
                $output_file_name = $MadelineProto->download_to_dir($MessageMedia, 'tmp');
                echo $output_file_name;
            }
        }
    }