private function _resize($file_name, $size) {
        $this->load->library( 'image_lib' );

        $config['image_library'] = 'gd2';
        $config['source_image'] = 'img/upload/' . $file_name;
        $config['new_image'] = 'img/upload/' . $file_name;
        $config['create_thumb'] = TRUE;
        $config['thumb_marker'] = '_' . $size;
        $config['maintain_ratio'] = FALSE;
        $config['width'] = $size;
        $config['height'] = $size;

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

        if (!$this->image_lib->resize()) {
            echo $this->image_lib->display_errors();
        }
        $this->image_lib->clear();
        return;
    }