merge two png pic

[cocobear]

This two png file has their own palette

>>> im1.mode
'P'
>>> im.mode
'P'
>>> im.getpalette == im1.getpalette
False

I can use this code to merge two png pic together:

Map = Image.new("RGB", (x,y))
Map.paste(im, box)
Map.paste(im1,box)

Map = Map.convert("L", optimize=True, palette=Image.ADAPTIVE)

But if the two png pic is too big , or if I have to merge more pic
together, I will get MemoryError:

>>> Image.new("RGB",(44544,38656))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/PIL/Image.py", line 1710, in
new
    return Image()._new(core.fill(mode, size, color))
MemoryError

How can I directly merge two pic to a ‘P' mode png with palette.