The method described in the accepted answer didn't work for me even after applying changes mentioned in its comments. But the below simple code worked:
import matplotlib.pyplot as pltplt.imsave(filename, np_array, cmap='Greys')np_array could be either a 2D array with values from 0..1 floats o2 0..255 uint8, and in that case it needs cmap. For 3D arrays, cmap will be ignored.