What is the fastest way to draw an image from discrete pixel values in Python?

If you have numpy and scipy available (and if you are manipulating large arrays in Python, I would recommend them), then the scipy.misc.pilutil.toimage function is very handy.
A simple example:

import numpy as np
import scipy.misc as smp

# Create a 1024x1024x3 array of 8 bit unsigned integers
data = np.zeros( (1024,1024,3), dtype=np.uint8 )

data[512,512] = [254,0,0]       # Makes the middle pixel red
data[512,513] = [0,0,255]       # Makes the next pixel blue

img = smp.toimage( data )       # Create a PIL image
img.show()                      # View in default viewer

The nice thing is toimage copes with different data types very well, so a 2D array of floating-point numbers gets sensibly converted to grayscale etc.

You can download numpy and scipy from here. Or using pip:

pip install numpy scipy