Make PNG sprites in PIL but want to preserve alpha channel

I'm creating a script to grab a bunch of favicons, convert them to png and make a single sprite image out of all of them.

The helpful g.etfv.co will detect and convert favicons in PNG format, and I'm using PIL to montage the images into a sprite sheet.

It all works fine except I seem to lose the alpha transparency channel when I do this.

Resulting image from the below script:

Update: Now creating image in mode 'RGBA' rather than 'RGB'. Resulting image is 开发者_Python百科now only partially borked:

Update: I note that it so happens that images it messes up on it reports as being RGB (not RGBA) prior to conversion.

>>> print url, img.getbands()
http://stackoverflow.com/ ('R', 'G', 'B')
http://www.google.com/ ('R', 'G', 'B')
http://blog.okcupid.com/ ('R', 'G', 'B', 'A')

What am I doing wrong?

import urllib
import PIL.Image
import StringIO

favicon_base = 'http://g.etfv.co/'

icons = """
http://stackoverflow.com/
http://www.google.com/
http://blog.okcupid.com/
"""

icons = [icon.strip() for icon in icons.strip().splitlines()]

left = 0
right = 16
upper = 0
lower = 16

inew = PIL.Image.new('RGBA',(len(icons)*16,16))

for url in icons:
    favicon = urllib.urlopen(favicon_base + urllib.quote(url) + '?defaulticon=lightpng').read()

    img = PIL.Image.open(StringIO.StringIO(favicon))
    img = img.convert('RGBA')

    bbox = (left, upper, right, lower)
    inew.paste(img, bbox, img)

    left += 16
    right += 16


inew.save(open('blah.png', 'wb'), 'png', optimize=1)

The result image inew has no alpha chanel.

UPDATE:

try

inew = PIL.Image.new('RGBA',(len(icons)*16,16))