Flutter Image object to ImageProvider

Call .image on your ImageProvider.

In your case:

Container(
  decoration: BoxDecoration(
    color: Colors.green,
    image: DecorationImage(
      image: img.image // <--- .image added here
    )
)

In this case you can use:

Image img = Image.memory(base64Decode(BASE64_STRING));

Container(
  decoration: BoxDecoration(
    color: Colors.green,
    image: DecorationImage(
      image: img.image // convert Image object to ImageProvider
    )
)

Please Use Uint8List and MemoryImage , see code snippet
You can copy paste run full code below
code snippet

import 'dart:typed_data';
...
Uint8List bytes = base64Decode(BASE64_STRING);
...
Container(
              height: 200,
              decoration: BoxDecoration(
                  color: Colors.green,
                  image: DecorationImage(
                      image: MemoryImage(bytes)
                  )
              )),

working demo

full code

import 'package:flutter/material.dart';
import 'dart:convert';
import 'dart:typed_data';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

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

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

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    String BASE64_STRING = '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';
    Image img = Image.memory(base64Decode(BASE64_STRING));
    Uint8List bytes = base64Decode(BASE64_STRING);
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Container(
              height: 200,
              decoration: BoxDecoration(
                  color: Colors.green,
                  image: DecorationImage(
                      image: MemoryImage(bytes)
                  )
              )),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

Although you can get an ImageProvide from an Image widget using myImageWidget.image, you might as well start with the ImageProvider directly:

final imageProvider = MemoryImage(bytes);

Image.memory() gives you an Image widget, but MemoryImage gives you the ImageProvider.

See my fuller answer here for network, asset, and file images.