code: def _get_owner_cover_photo_upload_server(session, group_id, crop_x=0, crop_y=0, crop_x2=795, crop_y2=200):
          
          group_id = abs(group_id)
          response = session.fetch("photos.getOwnerCoverPhotoUploadServer", group_id=group_id, crop_x=crop_x, crop_y=crop_y, crop_x2=crop_x2, crop_y2=crop_y2)
          return response['upload_url']
docs: https://vk.com/dev/photos.getOwnerCoverPhotoUploadServer