Go to => android/src/main/res/drawable.Paste your icon it should be white tansparent png format

Then add below code AndroidManifest.xml file

<meta-data
  android:name="com.google.firebase.messaging.default_notification_icon"
android:resource="@drawable/noti" />

Then modified some code your notification service class

var androidInitializationSettings = const AndroidInitializationSettings('@drawable/noti');

 AndroidNotificationDetails androidNotificationDetails = AndroidNotificationDetails(
        channel.id.toString(),
        channel.name.toString() ,
        channelDescription: 'your channel description',
        styleInformation: bigPictureStyle,
        importance: Importance.high,
        priority: Priority.high ,
        playSound: true,
        ticker: 'ticker' ,
       sound: RawResourceAndroidNotificationSound(''),
       icon: '@drawable/noti',
    );

I hope it will work In Sha Allah :-)