CodeIgniter水印

嗨，我的代码有问题

    $obj = new stdClass();
    $obj->cat_id = !empty($_POST['cat_id']) ? $_POST['cat_id'] : '';
    $obj->num_chapter = !empty($_POST['num_chapter']) ? $_POST['num_chapter'] : '';
    $obj->tags = !empty($_POST['tags']) ? $_POST['tags'] : '';
    $obj->date = date("Y-m-d H:i:s");


    $config['upload_path'] = $this->model_mangachapt->Add($obj);
    $config['encrypt_name'] = false;
    $config['allowed_types'] = "png|jpg|jpeg";
    $config['max_size'] = "1000000";

    $this->upload->initialize($config);

    $img_config['source_image'] = $this->upload->do_multi_upload('userfile');
    $img_config['image_library'] = 'gd2';
    $img_config['wm_type'] = 'overlay';
    $img_config['wm_overlay_path'] = './images/logo.gif';
    $img_config['wm_vrt_alignment'] = 'middle';
    $img_config['wm_hor_alignment'] = 'center';

    $this->image_lib->initialize($img_config);
    print $this->image_lib->watermark();

我想要的是更新图像后将徽标从网上发布。但是如果我上传我会给我错误

错误是：

A PHP Error was encountered
Severity: Warning
Message: imagecolorat() expects parameter 1 to be resource, boolean given
Filename: libraries/Image_lib.php
Line Number: 953