Flutter – How to implement swipe to previous page in Flutter

I'm not sure I understand your question, but if you want the image to fill the entire screen you can use a DecorationImage with a fit of BoxFit.cover.

class BaseLayout extends StatelessWidget{
  @override
  Widget build(BuildContext context){
    return Scaffold(
      body: Container(
        decoration: BoxDecoration(
          image: DecorationImage(
            image: AssetImage("assets/images/bulb.jpg"),
            fit: BoxFit.cover,
          ),
        ),
        child: null /* add child content here */,
      ),
    );
  }
}

For your second question, here is a link to the documentation on how to embed resolution-dependent asset images into your app.

Below worked perfectly with me in both Android and iOS, I used exit(0) from dart:io

import 'dart:io';

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new ... (...),
          floatingActionButton: new FloatingActionButton(
            onPressed: ()=> exit(0),
            tooltip: 'Close app',
            child: new Icon(Icons.close),
          ), 
    );
  }

UPDATE Jan 2019 Preferable solution is:

SystemChannels.platform.invokeMethod('SystemNavigator.pop');

As described here