image in flutter.txt

Image.asset('image name') //flutter code

Step 2. Now you can copy your image to images sub-folder. The path should look like assets/images/yourImage.
Step 3. Register the assets folder in pubspec.yaml file and update it. 

a) To add images, write the following code:

flutter:   
  assets:
        - assets/images/yourFirstImage.jpg
        - assets/image/yourSecondImage.jpg

Step 4. Insert the image code in the file, where you want to add the image.

Image.asset('assets/images/GeeksforGeeks.jpg')

Image.asset(
                'assets/images/geeksforgeeks.jpg',
                height: 400,
                width: 400,
              ),

https://www.geeksforgeeks.org/how-to-add-images-in-flutter-app/