displaying BLOB data from DB

[eluser]junaids[/eluser]
i have changed th controller a bit

Code:

function image()
          {
             $u = $this->session->userdata('username');
             $this->db->where('username', $u);
            $this->db->select('Profile_picture');
            $image_data = $this->db->get('user');
            $image = imagecreatefromstring($image_data);
              header('Content-Type: image/png');
            imagepng($image);
            imagedestroy($image);
            exit;
          }

where user is the DB table and Profile_picture is the user table field.
now when i go to this function in address abr it gives many errors like.
Severity: 4096

Message: Object of class CI_DB_mysql_result could not be converted to string.
where m i wrong?