My solution to http://codegolf.stackexchange.com/q/33172

pixel_relocate.py

from PIL import Image
from optparse import OptionParser


def key_func(arr):
    # Sort the pixels by luminance
    r = 0.2126*arr[0] + 0.7152*arr[1] + 0.0722*arr[2]
    return r


def main():
    # Parse options from the command line
    parser = OptionParser()
    parser.add_option("-p", "--pixels", dest="pixels",
                      help="use pixels from FILE", metavar="FILE")
    parser.add_option("-i", "--input", dest="input", metavar="FILE",
                      help="recreate FILE")
    parser.add_option("-o", "--out", dest="output", metavar="FILE",
                      help="output to FILE", default="output.png")

    (options, args) = parser.parse_args()

    if not options.pixels or not options.input:
        raise Exception("Missing arguments. See help for more info.")

    # Load the images
    im1 = Image.open(options.pixels)
    im2 = Image.open(options.input)

    # Get the images into lists
    px1 = list(im1.getdata())
    px2 = list(im2.getdata())
    w1, h1 = im1.size
    w2, h2 = im2.size

    if w1*h1 != w2*h2:
        raise Exception("Images must have the same number of pixels.")

    # Sort the pixels lists by luminance
    px1_s = sorted(px1, key=key_func)
    px2_s = sorted(px2, key=key_func)

    # Create an array of nothing but black pixels
    arr = [(0, 0, 0)]*w2*h2

    # Loop through each value of the sorted pixels
    for index, val in enumerate(px2_s):
        # Find the original location of the pixel
        v = px2.index(val)
        # Set the value of the array at the given location to the pixel of the
        # equivalent luminance from the source image
        arr[v] = px1_s[index]
        # Set the value of px2 to an arbitrary value outside of the RGB range
        # This prevents duplicate pixel locations
        # I would use "del px2[v]", but it wouldn't work for some reason
        px2[v] = (512, 512, 512)
        # Print the percent progress
        print("%f%%" % (index/len(px2)*100))

    # Save the image
    im3 = Image.new('RGB', im2.size)
    im3.putdata(arr)
    im3.save(options.output)

if __name__ == '__main__':
    main()