make half-size version of images. (a.png -> a.png(/2), a@2x.png(original))

png-reduce.py

import Image
import os
import shutil
import math

def half(size):
    return int(math.ceil(size / 2.0))

class Candidate(str):
    SUFFIX = '@2x.png'
    @property
    def filename_for_double_size(self):
        return self[:-4] + self.SUFFIX

    def is_target(self):
        if not self.endswith('.png'):
            return False
        if self.endswith(self.SUFFIX):
            return False
        if os.path.exists(self.filename_for_double_size):
            return False
        return True

    def copy2x(self):
        shutil.copy(self, self.filename_for_double_size)

    def reduce_inplace(self):
        im = Image.open(self)
        size = im.size
        newsize = map(half, size)
        im = im.resize(newsize)
        im.save(self)
        return size, newsize

for p, ds, fs in os.walk('.'):
    for fn in fs:
        candidate = Candidate(os.path.join(p, fn))
        if candidate.is_target():
            print candidate
            candidate.copy2x()
            size, newsize = candidate.reduce_inplace()
            print '    {0[0]:4}x{0[1]:4} -> {1[0]:4}x{1[1]:4}'.format(size, newsize)