您可以将资产图像转换为文件，然后上传到Firebase！下面是要转换的代码：

import 'dart:async';
import 'dart:io';

import 'package:flutter/services.dart' show rootBundle;
import 'package:path_provider/path_provider.dart';

Future<File> getImageFileFromAssets(String path) async {
  final byteData = await rootBundle.load('assets/$path');

  final file = File('${(await getTemporaryDirectory()).path}/$path');
  await file.writeAsBytes(byteData.buffer.asUint8List(byteData.offsetInBytes, byteData.lengthInBytes));

  return file;
}

在你的例子中，你可以这样调用这个函数：
File f = await getImageFileFromAssets('images/myImage.jpg');
编辑你的代码：

File avatarImageFile;

    Future getImage() async {
        File f = await getImageFileFromAssets('path of your asset Image');
        File image = await ImagePicker.pickImage(source: ImageSource.gallery);

      if (image != null) {
        setState(() {
          avatarImageFile = image;
          isLoading = true;
        });
      }else{
        avatarImageFile = f;
        isLoading = true;
      }

        uploadFile();
      }