How do I add a background image to flutter app?

Scaffold doesn’t support any concept of a background image. What you can do is give the Scaffold a transparent color and put it in a Container and use the decoration property to pull in the required background image. The app bar is also transparent.

Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Welcome to Flutter',
      home: Container(
        decoration: BoxDecoration(
            image: DecorationImage(
                image: AssetImage("images/logo.png"), fit: BoxFit.cover)),
        child: Scaffold(
          backgroundColor: Colors.transparent,
          appBar: AppBar(
            elevation: 0,
            backgroundColor: Colors.transparent,
            title: Text('My App'),
            centerTitle: true,
            leading: IconButton(
                icon: Icon(
                  Icons.list,
                  color: Colors.white,
                ),
                onPressed: () {}),
          ),
        ),
      ),
    );
  }