_uploadImage

private function _uploadImage()
{
    $config['upload_path']          = './upload/product/';
    $config['allowed_types']        = 'gif|jpg|png';
    $config['file_name']            = $this->product_id;
    $config['overwrite']            = true;
    $config['max_size']             = 1024; // 1MB
    // $config['max_width']            = 1024;
    // $config['max_height']           = 768;

    $this->load->library('upload', $config);

    if ($this->upload->do_upload('image')) {
        return $this->upload->data("file_name");
    }

    return "default.jpg";
}