A Flutter plugin for Android and iOS supports cropping images

<activity
    android:name="com.yalantis.ucrop.UCropActivity"
    android:screenOrientation="portrait"
    android:theme="@style/Theme.AppCompat.Light.NoActionBar"/>

Future<Null> _cropImage(File imageFile) async {
    File croppedFile = await ImageCropper.cropImage(
      sourcePath: imageFile.path,
      ratioX: 1.0,
      ratioY: 1.0,
      maxWidth: 512,
      maxHeight: 512,
    );
}