code: public GetDocumentImagesResponse getDocumentImages(String documentId) {
          checkNotNull(documentId, "documentId should not be null.");
          GetDocumentImagesRequest request = new GetDocumentImagesRequest();
          request.setDocumentId(documentId);
          InternalRequest internalRequest = this.createRequest(HttpMethodName.GET, request, DOC, request.getDocumentId());
          internalRequest.addParameter("getImages", null);
          GetDocumentImagesResponse response;
          try {
              response = this.invokeHttpClient(internalRequest, GetDocumentImagesResponse.class);
          } finally {
              try {
                  internalRequest.getContent().close();
              } catch (Exception e) {
                  // ignore exception
              }
          }
  
          return response;
      }
docs: get a Document Image list if Converted to image.
  Make Sure the Document convert type is image, otherwise will throw BceServiceException
  
  @param documentId the documentId need to get.
  
  @return A GetDocumentImageResponse object containing the information returned by Document.