BitmapImage bmp = new BitmapImage(new Uri(@"C:UsersadminDesktopbg.png"));
WriteableBitmap wrb = new WriteableBitmap(bmp);

byte[] color = new byte[] { 255, 0, 0, 255 };

wrb.WritePixels(new Int32Rect(0, 0, 1, 1), color, 4, 0);

image.Source = wrb;