def image(self, instance, disk_name, image_type=None):
        """Constructs image for selected backend

        :instance: Instance name.
        :name: Image name.
        :image_type: Image type.
                     Optional, is CONF.libvirt.images_type by default.
        """
        backend = self.backend(image_type)
        return backend(instance=instance, disk_name=disk_name)