Flutter: Adding background image with image picker

You can copy paste run full code below
You can check _image == null and to show TransparentImage or FileImage
code snippet

import 'package:transparent_image/transparent_image.dart';
...
body: Container(
          decoration: BoxDecoration(
        image: DecorationImage(
          image: _image == null
              ? MemoryImage(kTransparentImage)
              : FileImage(_image),
        ),
      )),

working demo

full code

import 'dart:io';

import 'package:flutter/material.dart';
import 'package:image_picker/image_picker.dart';
import 'package:transparent_image/transparent_image.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  File _image;
  final picker = ImagePicker();

  Future _getImage() async {
    final pickedFile = await picker.getImage(source: ImageSource.gallery);
    setState(() {
      if (pickedFile != null) {
        _image = File(pickedFile.path);

        print('_image: $_image');
      } else {
        print('No image selected');
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Container(
          decoration: BoxDecoration(
        image: DecorationImage(
          image: _image == null
              ? MemoryImage(kTransparentImage)
              : FileImage(_image),
        ),
      )),
      floatingActionButton: FloatingActionButton(
        onPressed: _getImage,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}