使用透明度Codeigniter上传图像

Question

Hi i have a problem with upload on codeigniter 3.0.

The image save with black background.. why ??

code:

 $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;

I don't understand why happen it. Please help me