使用透明度Codeigniter上传图像

Question

嗨，我在codeigniter 3.0上上传时遇到问题。

图像保存为黑色背景。为什么？

代码：

 $this->load->library('upload', $config);
            if ($this->upload->do_upload("immagine")) :
                $this->load->library("image_lib");
                $file = $this->upload->data();
                $nome_file_originale = $file["file_name"];
                $config = array();
                $config['source_image'] = "../public/offerte/".$nome_file_originale;
                //ImageJPEG(ImageCreateFromString(file_get_contents($config['source_image'])), $config['source_image'], 70);

                // thumb
                $config['imagealphablending']  = TRUE;
                $config['imagesavealpha']  = TRUE;
                $config['create_thumb'] = TRUE;
                $config['thumb_marker'] = '__thumbnail';
                $config['maintain_ratio'] = FALSE;
                $config['width'] = 400;
                $config['height'] = 256;
                $this->image_lib->initialize($config);
                $data["immagine_th"] = $this->image_lib->resize();
                $this->image_lib->clear();
                // large
                $config['imagealphablending']  = TRUE;
                $config['imagesavealpha']  = TRUE;
                $config['create_thumb'] = TRUE;
                $config['maintain_ratio'] = FALSE;
                $config['width'] = 800;
                $config['height'] = 800;
                $this->image_lib->initialize($config);
             $data["immagine_zoom"] = $this->image_lib->resize();
                $this->image_lib->clear();

endif;

我不明白为什么会这样。 请帮助我