A quick and dirty snippet so I don’t have to constantly look this up!

  BitmapSource imageSource = ...

 using (FileStream stream = new FileStream(@"...\Test.png", FileMode.Create)) {
     PngBitmapEncoder encoder = new PngBitmapEncoder();
     encoder.Interlace = PngInterlaceOption.On;
     encoder.Frames.Add(BitmapFrame.Create(imageSource));
     encoder.Save(stream);
 }