PIL transparency doesn't seem to work

Question

I'm trying to make the black pixels in the background of an image transparent. For this, I wrote the function below. I then overlay the image (a car in the center surrounded by black pixels) on a copy of itself and move the first one slowly. I was expecting the first image to reveal the copy underneath without showing the rectangular boundary since that part should have been transparent. However, I don't get the desired effect. Does anyone know what I'm missing?

def makeImageTransparent(img):
  img = img.convert("RGBA")
  datas = img.getdata()
  newData = []
  ii = 0
  for item in datas:
    if item[0] == 0 and item[1] == 0 and item[2] == 0:
        newData.append((0, 0, 0, 0))
        ii = ii + 1
    else:
        newData.append(item)
  print str(ii)
  img.putdata(newData)
  return img

Answer 1

I couldn't figure out how to make the transparency work, so I just created my own paste method that updated the bytes of the image I was pasting to directly and this got me the desired effect.

def pasteImage(img, bigim, posn):
  pixdata = img.load()
  width, height = img.size
  mainpixdata = bigim.load()
  for y in xrange(height):
    for x in xrange(width):
        if pixdata[x, y] != (0, 0, 0, 0):
            mainpixdata[x+posn[0], y+posn[1]] = pixdata[x,y]