I used Retrofit 2.0 for my register users, send multipart/form File image and text from register account
In my RegisterActivity, use an AsyncTask
//AsyncTask
private class Register extends AsyncTask<String, Void, String> {
    @Override
    protected void onPreExecute() {..}
    @Override
    protected String doInBackground(String... params) {
        new com.tequilasoft.mesasderegalos.dbo.Register().register(txtNombres, selectedImagePath, txtEmail, txtPassword);
        responseMensaje = StaticValues.mensaje ;
        mensajeCodigo = StaticValues.mensajeCodigo;
        return String.valueOf(StaticValues.code);
    }
    @Override
    protected void onPostExecute(String codeResult) {..}
And in my Register.java class is where use Retrofit with synchronous call
import android.util.Log;
import com.tequilasoft.mesasderegalos.interfaces.RegisterService;
import com.tequilasoft.mesasderegalos.utils.StaticValues;
import com.tequilasoft.mesasderegalos.utils.Utilities;
import java.io.File;
import okhttp3.MediaType;
import okhttp3.MultipartBody;
import okhttp3.RequestBody;
import okhttp3.ResponseBody;
import retrofit2.Call; 
import retrofit2.Response;
/**Created by sam on 2/09/16.*/
public class Register {
public void register(String nombres, String selectedImagePath, String email, String password){
    try {
        // create upload service client
        RegisterService service = ServiceGenerator.createUser(RegisterService.class);
        // add another part within the multipart request
        RequestBody requestEmail =
                RequestBody.create(
                        MediaType.parse("multipart/form-data"), email);
        // add another part within the multipart request
        RequestBody requestPassword =
                RequestBody.create(
                        MediaType.parse("multipart/form-data"), password);
        // add another part within the multipart request
        RequestBody requestNombres =
                RequestBody.create(
                        MediaType.parse("multipart/form-data"), nombres);
        MultipartBody.Part imagenPerfil = null;
        if(selectedImagePath!=null){
            File file = new File(selectedImagePath);
            Log.i("Register","Nombre del archivo "+file.getName());
            // 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
            imagenPerfil = MultipartBody.Part.createFormData("imagenPerfil", file.getName(), requestFile);
        }
        // finally, execute the request
        Call<ResponseBody> call = service.registerUser(imagenPerfil, requestEmail,requestPassword,requestNombres);
        Response<ResponseBody> bodyResponse = call.execute();
        StaticValues.code  = bodyResponse.code();
        StaticValues.mensaje  = bodyResponse.message();
        ResponseBody errorBody = bodyResponse.errorBody();
        StaticValues.mensajeCodigo  = errorBody==null
                ?null
                :Utilities.mensajeCodigoDeLaRespuestaJSON(bodyResponse.errorBody().byteStream());
        Log.i("Register","Code "+StaticValues.code);
        Log.i("Register","mensaje "+StaticValues.mensaje);
        Log.i("Register","mensajeCodigo "+StaticValues.mensaje);
    }
    catch (Exception e){
        e.printStackTrace();
    }
}
}
In the interface of RegisterService
public interface RegisterService {
@Multipart
@POST(StaticValues.REGISTER)
Call<ResponseBody> registerUser(@Part MultipartBody.Part image,
                                @Part("email") RequestBody email,
                                @Part("password") RequestBody password,
                                @Part("nombre") RequestBody nombre
);
}
For the Utilities parse ofr InputStream response
public class Utilities {
public static String mensajeCodigoDeLaRespuestaJSON(InputStream inputStream){
    String mensajeCodigo = null;
    try {
        BufferedReader reader = new BufferedReader(
                new InputStreamReader(
                    inputStream, "iso-8859-1"), 8);
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) {
            sb.append(line).append("\n");
        }
        inputStream.close();
        mensajeCodigo = sb.toString();
    } catch (Exception e) {
        Log.e("Buffer Error", "Error converting result " + e.toString());
    }
    return mensajeCodigo;
}
}