code: public Gravatar setCustomDefaultImage(String customDefaultImage,
                                            String encoding)
              throws UnsupportedEncodingException {
          assert customDefaultImage != null && encoding != null;
  
          this.customDefaultImage = URLEncoder.encode(customDefaultImage,
                  encoding);
  
          standardDefaultImage = null;
          return this;
      }
docs: Set the default image which will be retrieved when there is no avatar
  for the given email, when the avatar can't be shown due to the rating
  or when you enforce the default avatar.
  
  @param customDefaultImage Absolute URL to an image.
  @param encoding customDefaultImage's (first parameter) encoding
  @return Fluent interface
  @see java.net.URLEncoder#encode(String, String)