Image.file()接受File类类型的属性，而ImagePicker().getImage()方法返回PickedFile类型。
我们必须使用返回的PickedFile参数的getter .path，并将该文件路径传递给create a File对象，如下所示：

void _setImage() async {
    final picker = ImagePicker();
    PickedFile pickedFile = await picker.getImage(source: ImageSource.gallery);
    imageFile = File(pickedFile.path);
}

这可以在一行中完成，如下所示：

void _setImage() async {
    imageFile = File(await ImagePicker().getImage(source: ImageSource.gallery).then((pickedFile) => pickedFile.path));
}

在此之后，您可以使用变量imageFile，并根据需要将其像Image.file(imageFile)一样传递到Image.file()内部，或者像FileImage(imageFile)一样传递到FileImage()内部。
有关更多信息，请参见image_picker documentation on pub.dev

//many time when user import dart.html package than it throw error so keep note that we have to import dart.io

import 'dart.io';

final imagePicker = ImagePicker();
File imageFile;

Future getImage() async {
  var image = await imagePicker.getImage(source: ImageSource.camera);
  setState(() {
    imageFile = File(image.path);
  });
}

将PickedFile imageSelected更改为File imageSelected，并使用ImagePicker.pickImage(source: ImageSource.gallery)代替picker.getImage(source: ImageSource.gallery);

import 'package:image_picker/image_picker.dart';
import 'dart:io';  

  var image;
      void imagem() async {
        PickedFile picked = await ImagePicker().getImage(
            preferredCameraDevice: CameraDevice.front, source: ImageSource.camera);
        setState(() {
          image = File(picked.path);
        });
      }

或者，如果您需要完整地代码：

import 'dart:io';
import 'package:flutter/material.dart';
import 'package:image_picker/image_picker.dart';

class Photos extends StatefulWidget {
  @override
  _PhotosState createState() => _PhotosState();
}

class _PhotosState extends State<Photos> {
  var image;
  void imagem() async {
    PickedFile picked = await ImagePicker().getImage(
        preferredCameraDevice: CameraDevice.front, source: ImageSource.camera);
    setState(() {
      image = File(picked.path);
    });
  }

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
        child: Center(
      child: Column(
        children: [
          RaisedButton(
            onPressed: imagem,
            child: Text("Imagem"),
          ),
          image != null ? Image.file(image) : Text("I")
        ],
      ),
    ));
  }
}

墨菲斯的回答是正确的。
只需将PickedFile变量路径传递给File()。
示例：

final picker = ImagePicker();
    PickedFile pickedFile = await picker.getImage(source: ImageSource.gallery);
    imageFile = File(pickedFile.path);

尝试将imageFile类型转换为PickedImage，并以类型Casting the imageFile to File返回文件，例如：- //在此处声明变量
拾取的图像图像文件;
在返回的时候：-
return图像文件（文件（图像文件路径），宽度：400，身高：400年）;
我个人面临这个问题，这个解决方案解决了它。

试试这边...

Future pickImageFromGallery() async {
    try {
      final pickedFile = await picker.pickImage(
    source: ImageSource.gallery, 
      );
      setState(() {
    widget.imageFile = File(pickedFile!.path);
      });
      if (pickedFile == null) {
    throw Exception('File is not available');
      }
    } catch (e) {
      print(e);
    }