How to add a Background Images to the Screen?

You can do this by using DecoratedBox Widget. The example code below. Note: You can observe that Child of DecoratedBox doesn't have any code. If you are trying this example, you need to add image to your project and also add some widgets to Child within DecoaratedBox.

  import 'package:flutter/material.dart';

  void main() => runApp(BackgroundScreen());

  class BackgroundScreen extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        title: 'Backgroun Image',
        home: Scaffold(
            body: DecoratedBox(
                position: DecorationPosition.background,
                decoration: BoxDecoration(
                  color: Colors.red,
                  image: DecorationImage(
                      image: AssetImage('Images/Landscape.jpg'),
                      fit: BoxFit.cover),
                ),
                child: 
                  //Rest of the widgets go here..
                )
                ),
      );
    }
  }

Thanks,