flutterEmpty/lib/components/background.dart

import 'package:flutter/material.dart';

class Background extends StatelessWidget {

  final Widget child;
  final Widget image;

  Background({Key key, this.child, this.image}) : super(key:key);

  @override
  Widget build(BuildContext context) {

    return Container(
      width: double.infinity,
      height: double.infinity,
      child: Stack(
        alignment: Alignment.center,
        children: [
          image,
          child
        ],
      ),
    );
  }
}