Drop shadow using distans from edge. This method is fast because shadow area is filled by one pass and flexible because attenuation can be represented by a function of distance from edge.

drop_shadow3.py

import sys
import os
import math
import Image

def create_shadow_image(image_size, shadow_size):
    mergin = (shadow_size, shadow_size)
    new_size = map(sum, zip(image_size, mergin, mergin))
    im = Image.new('RGBA', new_size, 0xFFFFFFFF)
    buf = im.load()
    for y in range(new_size[1]):
        for x in range(new_size[0]):
            if y < shadow_size:
                if x < shadow_size:
                    # upper left
                    d = (shadow_size - x) ** 2 + (shadow_size - y) ** 2
                elif x < image_size[0] + shadow_size:
                    # upper center
                    d = (shadow_size - y) ** 2
                else:
                    # upper right
                    d = (x - image_size[0] - shadow_size) ** 2 + (shadow_size - y) ** 2
            elif y < image_size[1] + shadow_size:
                if x < shadow_size:
                    # center left
                    d = (shadow_size - x) ** 2
                elif x < image_size[0] + shadow_size:
                    # center center
                    d = 0
                else:
                    # center right
                    d = (x - image_size[0] - shadow_size) ** 2
            else:
                if x < shadow_size:
                    # bottom left
                    d = (shadow_size - x) ** 2 + (y - image_size[1] - shadow_size) ** 2
                elif x < image_size[0] + shadow_size:
                    # bottom center
                    d = (y - image_size[1] - shadow_size) ** 2
                else:
                    # bottom right
                    d = (x - image_size[0] - shadow_size) ** 2 + (y - image_size[1] - shadow_size) ** 2
            t = math.sqrt(d) / math.sqrt(2 * shadow_size ** 2)
            v = int(255 * (1 - (1 - t) ** 3))
            buf[x, y] = (v, v, v, 128)
    sys.stdout.flush()
    return im

def draw_shadowed_image(filename):
    SHADOW_SIZE = 30
    im = Image.open(filename)
    new_im = create_shadow_image(im.size, SHADOW_SIZE)
    offset = (SHADOW_SIZE,) * 4
    base = (0, 0) + im.size
    new_im.paste(im, map(sum, zip(base, offset)))
    root, ext = os.path.splitext(filename)
    new_im.save(root + '_shadowed' + ext)

if __name__ == '__main__':
    if len(sys.argv) < 2:
        sys.stderr.write('filename please!\n')
        sys.exit(1)
    draw_shadowed_image(sys.argv[1])