You only insert white because of this line:
image[h, w] = new Bgr(255,255,255);
If you want to add black or white, then you have to get an intermediate random number:
int w = random.Next() % image.Width;
int h = random.Next() % image.Height;
int shade = (random.Next(0, 256) > 127) ? 255 : 0;
image[h, w] = new Bgr(shade, shade, shade);