Future userPasswordUpdate() async {
    String passwordU = password.text;
    String confirmPasswordU = confirmPassword.text;
    String oldPasswordU = oldPassword.text         
        For the image upload you can also use the Dio library to post image on server with requested parameters.Please check the below example of it.
Dio dio = new Dio(); // with default Options
// Set default configs
    dio.options.baseUrl = BASE_URL;
    dio.options.connectTimeout = 5000; //5s
    dio.options.receiveTimeout = 3000;
    dio.options.headers[HEADER_AUTH_TOKEN_KEY] = HEADER_AUTH_TOKEN_VALUE;
    dio.options.headers[HEADER_VERSION_KEY] = HEADER_VERSION_VALUE;
    FormData formData = new FormData.fromMap({
      "password": passwordU,
      "confirmPass": confirmPasswordU,
      "oldpassword": oldPasswordU,
      "YOUR_IMAGE_PARAMETER_NAME": await MultipartFile.fromFile(imageFile.path,filename: imageFile.path.split("/").last),
    });
    var response = await dio.post(REGISTRATION_URL, data: formData);
    if (response.statusCode == 200) {
      apiResponse.onSuccess(response.toString(), eventType);
      print("Image Uploaded");
    } else {
      apiResponse.onError('Failed to load post');
      print("Upload Failed");
    }
  }
Inside the pubspec.yaml used this library,
dio: ^3.0.9
For the info about the library , you need to check this link Click