//图片集合
List<String> imgs = new ArrayList<>();
//压缩后的图片路径集合
List<String> tmpImgs = new ArrayList<>();

Handler mHandler = new Handler() {
  @Override
  public void handleMessage(Message msg) {
    super.handleMessage(msg);
    //TODO 收到消息后调用网络请求上传
  }
};

public void compressImages() {
  new Thread(new Runnable() {
    @Override
    public void run() {
      for (String path : imgs) {
        //TODO 调用压缩图片的方法，压缩后保存在一个临时文件夹中
        tmpImgs.add("压缩后路径");
      }
      mHandler.sendEmptyMessage(0);
    }
  }).start();
}

@Multipart
@POST("your address")
Observable<String> uploadImgs(@PartMap Map<String, RequestBody> map,                     @Part("imgs") MultipartBody body);

//先定义一个请求接口，除了图片可能还有其他一些参数需要上传，所以还定义了个map。接下来开始正文：

public void upload() {
  final Map<String, RequestBody> map = new HashMap<>();
  map.put("userId", RequestBody.create(MediaType.parse("form-data"),"1");
  final MultipartBody.Builder builder = new MultipartBody.Builder();
  Observable.from(imgs)
      .map(new Func1<String, String>() {
        @Override
        public String call(String path) {
          //调用图片压缩，返回压缩后路径tmp_path
          //注意，Filedata是后台给你的对应的字段
          builder.addFormDataPart("Filedata", "avatar.png", RequestBody.create(MultipartBody.FORM, new File(tmp_path)));
          return path;
        }
      }).last()
      .flatMap(new Func1<String, Observable<String>>() {
        @Override
        public Observable<String> call(String path) {
          return apiService.uploadImgs(map, builder.build());
        }
      })
      .subscribeOn(Schedulers.io())
      .observeOn(AndroidSchedulers.mainThread())
      .subscribe(new Subscriber<String>() {
        @Override
        public void onCompleted() {
        }
        @Override
        public void onError(Throwable e) {
          //错误处理
        }
        @Override
        public void onNext(String res) {
          //成功后处理
        }
      });
}