Bagaimana cara untuk Menambahkan fitur upload gambar

Assalamualaikum,.. tinggal ditambahkan saja disana 1 inputan unutk file :
<input type='file' class='form-control' name='gambar'>
 
lalu pada aksi simpan testimoni pada models/model_testimoni.php tambahkan aksi simpan gambar, jadinya :
function insert_testimoni(){
    $config['upload_path'] = 'asset/images/';
    $config['allowed_types'] = 'gif|jpg|png|JPG|JPEG';
    $config['max_size'] = '3000'; // kb
    $this->load->library('upload', $config);
    $this->upload->do_upload('gambar');
    $hasil=$this->upload->data();
    if ($hasil['file_name']==''){
        $datadb = array('id_konsumen'=>$this->session->id_konsumen,
                        'isi_testimoni'=>$this->input->post('testimoni'),
                        'aktif'=>'N',
                        'waktu_testimoni'=>date('Y-m-d H:i:s'));
    }else{
        $datadb = array('id_konsumen'=>$this->session->id_konsumen,
                        'isi_testimoni'=>$this->input->post('testimoni'),
                        'aktif'=>'N',
                        'gambar'=>$hasil['file_name'],
                        'waktu_testimoni'=>date('Y-m-d H:i:s'));
    }
    $this->db->insert('testimoni',$datadb);
}