Here there is the function:
See also Insert IMAGE BLOB in Oracle Database.

FUNCTION get_image_xmp_metadata (ID IN wh_timage.id_image%TYPE)
      RETURN VARCHAR2
   IS
      res         VARCHAR (4000);
      image_src   ORDSYS.ORDIMAGE;
      metav       xmlsequencetype;
   BEGIN
      res := '';

      SELECT im.image
        INTO image_src
        FROM wh_timage im
       WHERE im.id_image = ID;

      metav := image_src.getmetadata ('XMP');

      FOR i IN 1 .. metav.COUNT
      LOOP
         res := res || metav (i).getstringval ();
      END LOOP;

      RETURN res;
   END;