Combine images using Pillow and Python

[GMA]

Hello,

I'm trying to write a python script using Pillow :
count files ( png images ) number create an image for every 64 images. example: if the folder containing 640 images ( or any other number ), I will get 10 images as a thumbnail of 64 images/outputimage

any idea?

I started by this step :

import os
import os.path
from PIL import Image


image_dir = os.path.abspath("path_to_png_folder")
# list all files in directory
files = os.listdir(image_dir)
# get all PNGs
png_files = filter(lambda x: x.endswith(".png"), files)
# make file paths absolute
image_files = map(lambda x: os.sep.join([image_dir, x]), png_files)

print (image_files)

n_files = len(image_files)
# print (n_files)

target_img = None
n_targets = 0
collage_saved = False
for n in range(n_files):
    img = Image.open(image_files[n])
    img.thumbnail((100, 100))

    if n % 64 == 0:
        # create an empty image for a collage
        target_img = Image.new("RGB", (800, 800))
        n_targets += 1
        collage_saved = False

    # paste the image at the correct position
    i = int(n / 8)
    j = n % 8
    target_img.paste(img, (100*i, 100*j))

    if (n + 1) % 64 == 0 and target_img is not None:
        # save a finished 8x8 collage
        target_img.save("{0:04}.png".format(n_targets))
        collage_saved = True

# save the last collage
if not collage_saved:
    target_img.save("{0:04}.png".format(n_targets))

I got 10 images, the first one contain 8x8 images, the others are empty ( black ). also, since my png files are a sequence from 0.png to 640.png; have any idea how to let the paste operation take images by order.
example: out1.pan will contain 0.png to 63.png

Thank you