Converting a bitmap to monochrome

I am trying to save an image as monochrome (black&white, 1 bit-depth) but I'm coming up lost how to do it.

I am starting with a png and converting to a bitmap for printing (it's a thermal printer and only supports black anyway - plus its slow as hell for large images if I try to send them as color/grayscale).

My code so far is dead simple to convert it to a bitmap, but it is retaining the original colour depth.

Image image = Image.FromFile("C:\\test.png");

byte[] bitmapFileData = null;
int bitsPerPixel = 1;
int bitmapDataLength;

using (MemoryStream str = new MemoryStream())
{
    image.Save(str, ImageFormat.Bmp);
    bitmapFileData = str.ToArray();
}