How to separate the directory for image and image thumbnail when the image is uploaded?

Code:
function do_upload(){

        $n = rand(0,100000);

        $nama_file   = $_FILES['userfile']['name'];

        $config['upload_path'] = './uploads/';

        $config['allowed_types'] = 'gif|jpg|png';

        $config['max_size'] = '100';

        $config['max_width'] = '1024';

        $config['max_height'] = '768';

        $config['encrypt_name'] = "$n";

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

        if ( ! $this->upload->do_upload())

            {

                $error = array('error' => $this->upload->display_errors());

            

        $this->load->view('admin/employeeAdd', $error);

        }    

    else

        {

        $data = array('upload_data' => $this->upload->data());

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

                $file_name = $image_data['file_name'];

                $this->Model_admin->imageUpload($file_name);

                

                $config['image_library'] = "gd2";

                $config['source_image'] = "./uploads/$file_name";

                $config['create_thumb'] = TRUE;

                $config['maintain_ration'] = TRUE;

                $config['width'] = 50;

                $config['height'] = 50;

                // in here I put the second of $config['upload_path']

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

                $this->image_lib->resize();

                

        echo "Register Successfull<br />";

                echo anchor('admin/employee', 'Back');

        }

    }