PIL transparent image

# -*- coding: utf-8 -*-
import os
import Image

IN_DIR = "C:/Users/Karolis/Dropbox/rom/static/images/players"  # Absolute path of directory with images.
OUT_DIR = "transparent_images"              # Name of directory with output contents.
PIXELS_TO_CONVERT = (255, 255, 255)         # Pixels to be rewritten to transparent colour.

os.chdir(IN_DIR)                    # Moving to input directory.
try:
    os.mkdir(OUT_DIR)               # Creating output directory.
except WindowsError:
    pass
for f in os.listdir(os.getcwd()):
    if not ".." in f:
        img = Image.open(f)             # Opening image.
        img = img.convert("RGBA")       # Converting to RGBA format.

        pixels = img.load()             # Pixel map.

        # Looping through pixels and making them transparent where relevant.
        for y in xrange(img.size[1]):
            for x in xrange(img.size[0]):
                if pixels[x, y] == PIXELS_TO_CONVERT:
                    pixels[x, y] = (255, 255, 255, 0)

        os.chdir(OUT_DIR)       # Moving to output directory.
        img.save(f, "PNG")      # Saving transparent image.
        os.chdir(IN_DIR)        # Returning back to input directory.