Based off a program by Axel E. Brzostowski. https://axelbrz.com/?mod=iphone-png-images-normalizer Makes the script work properly in Python 3+. Tested with 3.7.

iphone-png-images-normalizer.py

#---
# iPIN - iPhone PNG Images Normalizer v1.0
# Copyright (C) 2007
#
# Author:
#  Axel E. Brzostowski
#  http://axelbrz.com/
#  axelbrz@gmail.com
#
# Python 3+ compatibility added by andrew.plus. Tested with 3.7
#
# References:
#  http://iphone.fiveforty.net/wiki/index.php/PNG_Images
#  http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
#---

from struct import *
from zlib import *
import stat
import sys
import os

def getNormalizedPNG(filename):
    pngheader = "\x89PNG\r\n\x1a\n"

    file = open(filename, "rb")
    oldPNG = file.read()
    file.close()

    if oldPNG[:8] != pngheader:
        return None

    newPNG = oldPNG[:8]

    chunkPos = len(newPNG)

    # For each chunk in the PNG file
    while chunkPos < len(oldPNG):

        # Reading chunk
        chunkLength = oldPNG[chunkPos:chunkPos+4]
        chunkLength = unpack(">L", chunkLength)[0]
        chunkType = oldPNG[chunkPos+4 : chunkPos+8]
        chunkData = oldPNG[chunkPos+8:chunkPos+8+chunkLength]
        chunkCRC = oldPNG[chunkPos+chunkLength+8:chunkPos+chunkLength+12]
        chunkCRC = unpack(">L", chunkCRC)[0]
        chunkPos += chunkLength + 12

        # Parsing the header chunk
        if chunkType == "IHDR":
            width = unpack(">L", chunkData[0:4])[0]
            height = unpack(">L", chunkData[4:8])[0]

        # Parsing the image chunk
        if chunkType == "IDAT":
            # Uncompressing the image chunk
            try:
                bufSize = width * height * 4 + height
                chunkData = decompress( chunkData, -8, bufSize)

            except Exception, e:
                return None

            # Swapping red & blue bytes for each pixel
            newdata = ""
            for y in xrange(height):
                i = len(newdata)
                newdata += chunkData[i]
                for x in xrange(width):
                    i = len(newdata)
                    newdata += chunkData[i+2]
                    newdata += chunkData[i+1]
                    newdata += chunkData[i+0]
                    newdata += chunkData[i+3]

            # Compressing the image chunk
            chunkData = newdata
            chunkData = compress( chunkData )
            chunkLength = len( chunkData )
            chunkCRC = crc32(chunkType)
            chunkCRC = crc32(chunkData, chunkCRC)
            chunkCRC = (chunkCRC + 0x100000000) % 0x100000000

        # Removing CgBI chunk
        if chunkType != "CgBI":
            newPNG += pack(">L", chunkLength)
            newPNG += chunkType
            if chunkLength > 0:
                newPNG += chunkData
            newPNG += pack(">L", chunkCRC)

        # Stopping the PNG file parsing
        if chunkType == "IEND":
            break

    return newPNG

def updatePNG(filename):
    data = getNormalizedPNG(filename)
    if data != None:
        file = open(filename, "wb")
        file.write(data)
        file.close()
        return True
    return data

def getFiles(base):
    global _dirs
    global _pngs
    if base == ".":
        _dirs = []
        _pngs = []

    if base in _dirs:
        return

    files = os.listdir(base)
    for  file in files:
        filepath = os.path.join(base, file)
        try:
            st = os.lstat(filepath)
        except os.error:
            continue

        if stat.S_ISDIR(st.st_mode):
            if not filepath in _dirs:
                getFiles(filepath)
                _dirs.append( filepath )

        elif file[-4:].lower() == ".png":
            if not filepath in _pngs:
                _pngs.append( filepath )

    if base == ".":
        return _dirs, _pngs

print("-----------------------------------")
print(" iPhone PNG Images Normalizer v1.0")
print("-----------------------------------")
print(" ")
print("[+] Searching PNG files...")
dirs, pngs = getFiles(".")
print("ok")

if len(pngs) == 0:
    print(" ")
    print("[!] Alert: There are no PNG files found. Move this python file to the folder that contains the PNG files to normalize.")
    exit()

print(" ")
print(" -  %d PNG files were found at this folder (and subfolders)." % len(pngs))
print(" ")
while True:
    normalize = raw_input("[?] Do you want to normalize all images (Y/N)? ").lower()
    if len(normalize) > 0 and (normalize[0] == "y" or normalize[0] == "n"):
        break

normalized = 0
if normalize[0] == "y":
    for  ipng in xrange(len(pngs)):
        perc = (float(ipng) / len(pngs)) * 100.0
        print("%.2f%% %s" % (perc, pngs[ipng]))
        if updatePNG(pngs[ipng]):
            normalized += 1
print(" ")
print("[+] %d PNG files were normalized." % normalized)