How can I read (from disk) and resize an image, in Flutter/Dart

You can read image from the disk using the image.file constructor.

For more features you can use the Image library

A Dart library providing the ability to load, save and manipulate
images in a variety of different file formats.

Sample from the documentation examples

Load a jpeg, resize it and save it as a png

    import 'dart:io' as Io;
    import 'package:image/image.dart';
    void main() {
      // Read a jpeg image from file.
      Image image = decodeImage(new Io.File('test.jpg').readAsBytesSync());

      // Resize the image to a 120x? thumbnail (maintaining the aspect ratio).
      Image thumbnail = copyResize(image, width: 120);

      // Save the thumbnail as a PNG.
      new Io.File('out/thumbnail-test.png')
            ..writeAsBytesSync(encodePng(thumbnail));
    }