docs: def _gather_image_parts(self):
          """Load the image part collection with all the image parts in package."""
          for rel in self.iter_rels():
              if rel.is_external:
                  continue
              if rel.reltype != RT.IMAGE:
                  continue
              if rel.target_part in self.image_parts:
                  continue
              self.image_parts.append(rel.target_part)