adding_border.dart

class _MyHomePageState extends State<MyHomePage> {
  String imageSource =
      'https://images.unsplash.com/photo-1494253109108-2e30c049369b?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1350&q=80';

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      backgroundColor: Colors.white,
      body: Center(
        child: Container(
          decoration: BoxDecoration(
            border: Border.all(
              color: Colors.orange,
              width: 20,
            ),
          ),
          child: Image.network(
            imageSource,
            width: 300,
            height: 300,
            fit: BoxFit.cover,
          ),
        ),
      ),
    );
  }
}