Поведение картинки при bottom / top

main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

final config = {
  'primaryColor': Colors.grey[200],
};

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primaryColor: config['primaryColor'],
        floatingActionButtonTheme: FloatingActionButtonThemeData(
          elevation: 35,
          highlightElevation: 20,
          foregroundColor: Colors.grey[800],
        ),
        accentColor: Colors.grey[200],
      ),
      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>
    with SingleTickerProviderStateMixin {
  Animation<double> animation;
  AnimationController controller;

  @override
  void initState() {
    super.initState();
    controller = AnimationController(
      duration: const Duration(seconds: 3),
      vsync: this,
      reverseDuration: Duration(seconds: 5),
    );
    animation = Tween<double>(begin: -80, end: -40).animate(controller)
      ..addListener(() {
        setState(() {});
      })
      ..addStatusListener((status) {
        if (status == AnimationStatus.completed)
          controller.reverse();
        else if (status == AnimationStatus.dismissed) controller.forward();
      });
    controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        elevation: 0,
        title: Text(widget.title),
      ),
      body: Stack(
        children: <Widget>[
          Positioned(
            bottom: 0,
            //bottom: 350,
            //bottom: 200, top: 200,
            child: Transform.translate(
              offset: Offset(animation.value, 15.0),
              child: Image.network(
  'https://b7.pngbarn.com/png/297/625/sculpture-figurine-michael-fallen-angel-angel-png-clip-art.png'),
            ),
          ),
        ],
      ),
      floatingActionButton: FloatingActionButton(
          child: Icon(Icons.ac_unit),
          onPressed: () {
            setState(() {
            });
          }),
    );
  }
}