使用codeigniter将图像插入到bdd中

I use the codeigniter framework. My image is stored in the bdd from my form but it does not show up. I suppose an encoding problem, when I put the form open multipart I have an error message that my image is NULL

le model

public function addPost(Articles_model $data){

   $this->load->helper('form');
    $data = array(

           'article_title' =>$data->getTitle(),
           'article_img' =>$this->input->post('article_img'),
           'article_content' =>$data->getContent(),
           'article_createdate' =>$data->getCreateDate(),


    );

     return $this->db->insert('articles',$data);


}

le controller

public function do_upload()
        {
                $config['upload_path']          = './uploads/';
                $config['allowed_types']        = '*';
                $config['max_size']             = 100;
                $config['max_width']            = 1024;
                $config['max_height']           = 768;

                $this->load->library('upload', $config);    
                $this->upload->initialize($config);
                if ( ! $this->upload->do_upload('userfile'))
                {
                        $error = array('error' => $this->upload->display_errors());

                        $this->load->view('upload_form', $error);
                }
                else
                {
                        $data = array('upload_data' => $this->upload->data());

                        $this->load->view('upload_success', $data);
                }
        }

the forms:

$data['form_open']          = form_open("/paging/addPost");//<form action="#" name="formCreateAccount" method="post" >
        $data['fieldset_user']      = form_fieldset($this->lang->line("users_infos")); //<fieldset><legend>Informations utilisateur</legend>
        $data['form_userid']        = form_hidden("id", $this->input->post('id'));//<input type="hidden" name="user_id" value="<?php echo isset($_POST['user_id'])?$_POST['user_id']:''

        // label + input titre
        $data['label_titre']            = form_label($this->lang->line("labeltitre"), 'titre'); //<label for="name">Nom</label>
        $data['form_titre']         = form_input('article_title', $this->input->post('article_title'), 'id="titre"');//<input required type="text" name="user_name" id="name" value="<?php echo isset($_POST['user_name'])?$_POST['user_name']:'';

        // label + input image
        $data['label_img']          = form_label($this->lang->line("article_imgs"), 'file'); //<label for="firstname">Prénom</label>
        $data['form_img']           = form_upload('article_img', $this->input->post('article_img'), 'id="img"');//<input required type="text" name="user_firstname" id="firstname" value="<?php echo isset($_POST['user_firstname'])?$_POST['user_firstname']:'';