imagecopy() expects parameter 1 to be resource, boolean given

Code:
   function do_upload_img()

    {

        if(isset($_FILES['portfolioimg']['size']) != 0){

           $name_array = array();

           $files = $_FILES;

           $cpt = count($_FILES['portfolioimg']['name']);



           // $config['source_image'] = ''; //get original image

           $config['wm_type'] = 'overlay';

           $config['wm_overlay_path'] = './assets/img/watermark.png';

           $config['quality'] = 50;

           $config['wm_vrt_alignment'] = 'middle';

           $config['wm_hor_alignment'] = 'center';

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



           for($i=0; $i<=$cpt-1; $i++)

           {           

               $_FILES['userfile']['name']= $files['portfolioimg']['name'][$i];

               $_FILES['userfile']['type']= $files['portfolioimg']['type'][$i];

               $_FILES['userfile']['tmp_name']= $files['portfolioimg']['tmp_name'][$i];

               $_FILES['userfile']['error']= $files['portfolioimg']['error'][$i];

               $_FILES['userfile']['size']= $files['portfolioimg']['size'][$i];    



               $imgnam = rand(10,1000000);

               $ext = pathinfo($files['portfolioimg']['name'][$i], PATHINFO_EXTENSION);

               if($this->upload->initialize($this->set_upload_options($imgnam))){

                   $data = $this->upload->do_upload();

                   $config['source_image'] = './assets/portfolio_img/' . 'Sooo_'.$imgnam.'.'.$ext;

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

                   $this->image_lib->watermark(); 

                   $name_array[] = $this->upload->data('file_name');

               } else {

                   $this->session->set_flashdata('falsemsg','Only jpg, jpeg, png files are allowed to be uploaded.');

               }

               

           } 

           return $name_array;

        }else{

          return false;

        }

    }