code: def normalise_filled(self, meta, val):
          
          available = list(meta.everything["images"].keys())
          val = sb.formatted(sb.string_spec(), formatter=MergedOptionStringFormatter).normalise(meta, val)
          if val not in available:
              raise BadConfiguration("Specified image doesn't exist", specified=val, available=available)
          return val
docs: Only care about valid image names