Parameters
        ----------
        identifier : string
            Unique image group identifier
        limit : int
            Limit number of results in returned object listing
        offset : int
            Set offset in list (order as defined by object store)

        Returns
        -------
        ObjectListing
            Listing of group images or None if image group does not exist

def list_images(self, identifier, offset=-1, limit=-1):
        """
        Parameters
        ----------
        identifier : string
            Unique image group identifier
        limit : int
            Limit number of results in returned object listing
        offset : int
            Set offset in list (order as defined by object store)

        Returns
        -------
        ObjectListing
            Listing of group images or None if image group does not exist
        """
        # Get image group to ensure that it exists. The object contains the full
        # list of group images
        img_grp = self.get_object(identifier)
        if img_grp is None:
            return None
        # Extract subset of group images based on offset and limit arguments
        total_count = len(img_grp.images)
        items = []
        if offset < total_count:
            if limit > 0:
                list_end = min((offset + limit), total_count)
            else:
                list_end = total_count
            for i in range(offset, list_end):
                items.append(img_grp.images[i])
        # Return object listing
        return datastore.ObjectListing(items, offset, limit, total_count)