mask.cs

        using (headshotStream)
        {
            using (var maskStream = File.Open(@"C:\Users\lbugn\Desktop\HeadshotMask.png", FileMode.Open))
            {
                var maskBitmap = SKBitmap.Decode(maskStream);
                var headshotBitmap = SKBitmap.Decode(headshotStream);

                using (var surface = SKSurface.Create(new SKImageInfo(1000, 1000)))
                {
                    var canvas = surface.Canvas;
                    canvas.DrawBitmap(maskBitmap, 0, 0);

                    using (var paint = new SKPaint())
                    {
                        paint.BlendMode = SKBlendMode.SrcIn;

                        canvas.DrawBitmap(headshotBitmap, new SKRect(0, 0, 1000, 1000), paint);

                        var final = surface.Snapshot();

                        using (var encoded = final.Encode(SKEncodedImageFormat.Png, 100))
                        {
                            using (var outputStream = File.OpenWrite(@"C:\Users\lbugn\Desktop\result.png"))
                            {
                                encoded.SaveTo(outputStream);
                            }
                        }
                    }
                }
            }
        }