It\'s embarrassing to ask this question but can\'t find an answer.
I tried this in vain.
Image resultImage = new Bitmap(image1.Width, image1.Height,
You are assigning a new image to resultImage, thereby overwriting your previous attempt at creating a white image (which should succeed, by the way).
resultImage
So just remove the line
resultImage = new Bitmap(image1.Width, image1.Height, grp);