public class MainActivity extends AppCompatActivity {

  private OkHttpClient client;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    client = new OkHttpClient();
    postImage();
  }

  private void postImage(){
    //这里方便演示。读取drawable里的图片。
    Bitmap bm = BitmapFactory.decodeResource(getResources(),R.drawable.demo);

    ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();

    bm.compress(Bitmap.CompressFormat.JPEG,100,byteArrayOutputStream);

    MultipartBody.Builder builder = new MultipartBody.Builder().setType(MultipartBody.FORM)
        .addFormDataPart("img_1","1.jpg", RequestBody.create(MediaType.parse("image/jpeg"),byteArrayOutputStream.toByteArray()))
        .addFormDataPart("img_2","2.jpg", RequestBody.create(MediaType.parse("image/jpeg"),byteArrayOutputStream.toByteArray()))
        ;
    //有多个图片就用for循环添加即可

    MultipartBody build = builder.build();

    okhttp3.Request bi = new okhttp3.Request.Builder()
        .url("后台地址") 
        .post(build)
        .build();

    client.newCall(bi).enqueue(new Callback() {
      @Override
      public void onFailure(Call call, IOException e) {
        Log.i("TAG", "onFailure: 失败");
      }

      @Override
      public void onResponse(Call call, okhttp3.Response response) throws IOException {
        Log.i("TAG", "onResponse: " + response.body().string());
        //提交成功处理结果....
      }
    });

  }
}

<?php
header('Content-type: application/json;charset=utf-8');

if(empty($_FILES)) die('{"status":0,"msg":"错误提交"}');

$dirPath = './img/';//设置文件保存的目录

if(!is_dir($dirPath)){
  //目录不存在则创建目录
  @mkdir($dirPath);
}

$count = count($_FILES);//所有文件数

if($count<1) die('{"status":0,"msg":"错误提交"}');//没有提交的文件

$success = $failure = 0;

foreach($_FILES as $key => $value){
  //循环遍历数据
  $tmp = $value['name'];//获取上传文件名
  $tmpName = $value['tmp_name'];//临时文件路径
  //上传的文件会被保存到php临时目录，调用函数将文件复制到指定目录
  if(move_uploaded_file($tmpName,$dirPath.date('YmdHis').'_'.$tmp)){
    $success++;
  }else{
    $failure++;
  }
}

$arr['status'] = 1;
$arr['msg']   = '提交成功';
$arr['success'] = $success;
$arr['failure'] = $failure;

echo json_encode($arr);