Convert 8 bit image to 24 bit

Question

So I am trying to convert my grayscale image (8 bit) to 24 bit with python. And I tried to do it with numpy.

But how do i save it as a .png image?

import numpy as np
img = np.array(NewGrayScaleImage, dtype=np.uint16) 
img *= 384 #256 for 16 bit