We are assuming you are already familiar with Retrofit. If not then first go to this. We have used multipart request for uploading image on server using Retrofit.

Following are the simple and easy steps for image uploading using Retrofit:-

Step 1: Add required dependency in your module level gradle file for using Retrofit

compile 'com.squareup.retrofit2:retrofit:2.0.2'

If you want to use GSON then add this dependency also

compile 'com.squareup.retrofit2:converter-gson:2.0.2'

Step 2: Create an interface that contain an abstract method with return type Call<ResponseBody>. In this example I am using two parameters in this method. You can change the number of parameter according to your requirement.

public interface PicUploadInterface {
    @Multipart
    @POST("test.php")
    Call<ResponseBody> upload(@Part("description") RequestBody description,
                              @Part MultipartBody.Part file);
}

Step 3: Create a class that contains following:-

• Base url of your server
• Instance of OkHttpClient.Builder
• Instance of Retrofit.Builder

public class ServiceGenerator {

    public static final String API_BASE_URL = "Your_Server_URL";

    private static OkHttpClient.Builder httpClient = new OkHttpClient.Builder();

    private static Retrofit.Builder builder =
            new Retrofit.Builder()
                    .baseUrl(API_BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create());

    public static <S> S createService(Class<S> serviceClass) {
        Retrofit retrofit = builder.client(httpClient.build()).build();
        return retrofit.create(serviceClass);
    }
}

Step 4: Now create an api manager class from where you will create a request, Inside your class create a method with whatever name you want (in my case it is uploadFile) do the following : 

• Create reference of the interface created in previous step.
• Create request body with media type multipart.
• Crate body for multipart request.
• Create Request body.
• Crate the instance of the call and 

public void uploadFile(File file) {
        // create upload service client
        PicUploadInterface service =
                ServiceGenerator.createService(PicUploadInterface.class);

        // use the FileUtils to get the actual file by uri
        //File file = FileUtils.getFile(this, fileUri);

        // create RequestBody instance from file
        RequestBody requestFile =
                RequestBody.create(MediaType.parse("multipart/form-data"), file);

        // MultipartBody.Part is used to send also the actual file name
        MultipartBody.Part body =
                MultipartBody.Part.createFormData("profilePicture", file.getName(), requestFile);

        // add another part within the multipart request
        String descriptionString = "hello, description goes here";
        RequestBody description =
                RequestBody.create(
                        MediaType.parse("multipart/form-data"), "");

        // finally, execute the request
        Call<ResponseBody> call = service.upload(description, body);
        call.enqueue(new Callback<ResponseBody>() {
            @Override
            public void onResponse(Call<ResponseBody> call,
                                   Response<ResponseBody> response) {
                Log.v("Upload", "success");
            }

            @Override
            public void onFailure(Call<ResponseBody> call, Throwable t) {
                Log.e("Upload error:", t.getMessage());
            }
        });
    }

Step 5: Call uploadFile method  of APIManager(It is singleton class which contains uploadFile method mentioned in step 4) in order upload image file and pass image file that has to be uploaded

ApiManager.getInstance().uploadFile(file);

  For queries, do let me know in comments.