Create RGB image (A replacing B channel) out of RGBA image.

gistfile1.py

from PIL import Image
import glob, os

for infile in glob.glob( "*.png" ):

    file, ext = os.path.splitext( infile )

    im = Image.open( infile )
    im.load()
    r, g, b, a = im.split()

    im2 = Image.merge("RGB", (r,g,a))
    im2.save( file + "_packed.png", "PNG" )