private Iterable<Image> listImages(String projectId) throws OpsException {
    List<Image> ret = Lists.newArrayList();

    ImageList imageList;
    try {
      log.debug("Listing images in project " + projectId);
      imageList = compute.images().list(projectId).execute();
    } catch (IOException e) {
      throw new OpsException("Error listing images", e);
    }
    if (imageList.getItems() != null) {
      ret.addAll(imageList.getItems());
    }

    return ret;
  }