code: public UploadImgResponse uploadImg(File file){
          UploadImgResponse response;
          String url = "https://api.weixin.qq.com/cgi-bin/media/uploadimg?access_token=#";
          BaseResponse r = executePost(url, null, file);
          response = JSONUtil.toBean(r.getErrmsg(), UploadImgResponse.class);
          return response;
      }
docs: 上传群发消息图片素材
  @param file 上传的图片
  @return 上传结果