Example of how to add a vignette style border to any widget using a CustomPainter and magical BlendMode's.

photo_border.dart

import 'package:flutter/material.dart';

void main() => runApp(PhotoTestApp());

class PhotoTestApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Container(
        color: Colors.red,
        child: Center(
          child: PhotoBorder.basic(
            amount: 30.0,
            child: Image.asset('assets/image.jpg'),
          ),
        ),
      ),
    );
  }
}

class PhotoBorder extends StatelessWidget {
  const PhotoBorder.basic({
    Key key,
    double amount,
    this.child,
  })  : this.borderDepth = amount * 2,
        this.borderBlur = amount,
        super(key: key);

  const PhotoBorder({
    Key key,
    @required this.borderDepth,
    @required this.borderBlur,
    this.child,
  }) : super(key: key);

  final double borderDepth;
  final double borderBlur;
  final Widget child;

  @override
  Widget build(BuildContext context) {
    return CustomPaint(
      foregroundPainter: _PhotoBorderPainter(this),
      child: child,
    );
  }
}

class _PhotoBorderPainter extends CustomPainter {
  const _PhotoBorderPainter(this.widget);

  final PhotoBorder widget;

  @override
  void paint(Canvas canvas, Size size) {
    final rect = (Offset.zero & size).deflate(widget.borderDepth);
    canvas.saveLayer((Offset.zero & size), Paint()..blendMode = BlendMode.dstATop);
    canvas.drawRect(
      rect,
      Paint()..maskFilter = MaskFilter.blur(BlurStyle.normal, widget.borderBlur),
    );
    canvas.restore();
  }

  @override
  bool shouldRepaint(_PhotoBorderPainter old) {
    return old.widget.borderBlur != widget.borderBlur || old.widget.borderDepth != widget.borderDepth;
  }
}