fail to load image in cezpdf codeigniter

Code:
function exportPDF() 

{

    $this->load->library('cezpdf');

    $this->load->helper('pdf');

    $this->cezpdf->cezpdf('a4','portrait');

    $data['unit_kerja']= $this->admin_model->getUnitKerjaPDF();

    $titlecolumn = array(

                        'id_indeks' => 'Nomor Indeks',

                        'nama_unit' => 'Nama Unit Kerja'

    );

    $this->cezpdf->ezTable($data['unit_kerja'],$titlecolumn,'Master Unit Kerja');

    $image=base_url().'images/bps.png';

    $this->cezpdf->ezImage($image);

    $this->cezpdf->ezSetMargins(50,50,50,50);

    $this->cezpdf->addText(50,32,8,'Printed on ' . date('m/d/Y h:i:s a'). ' copyright cmail system BPS');

    $this->cezpdf->ezStream();

}