在Flutter中使用BottomNavigationBar，我如何设置背景图像，而不是只使用颜色？

import 'package:flutter/material.dart';

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

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

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _currentIndex = 0;

  List<Widget> _pages = [
    Page1(),
    Page2(),
    Page3(),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: [
          Positioned.fill(
            child: Image.asset(
              'assets/background_image.jpg',
              fit: BoxFit.cover,
            ),
          ),
          _pages[_currentIndex],
        ],
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        onTap: (index) {
          setState(() {
            _currentIndex = index;
          });
        },
        items: [
          BottomNavigationBarItem(
            icon: Icon(Icons.home),
            label: 'Page 1',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.search),
            label: 'Page 2',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.person),
            label: 'Page 3',
          ),
        ],
      ),
    );
  }
}

class Page1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: Center(
        child: Text(
          'Page 1',
          style: TextStyle(fontSize: 24),
        ),
      ),
    );
  }
}

class Page2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: Center(
        child: Text(
          'Page 2',
          style: TextStyle(fontSize: 24),
        ),
      ),
    );
  }
}

class Page3 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: Center(
        child: Text(
          'Page 3',
          style: TextStyle(fontSize: 24),
        ),
      ),
    );
  }
}