New PIL user : How to merge images

[vincent_delft at yahoo.com]

Does any one can give me sample of PIL code that merge 2 pictures.

To "normalize" pictures size, I would like to superpose pictures on a "fixed
size" background.

Thanks




I've tried : 

import Image
bg=Image.open('back.jpg') # size 800,600
img=Image.open('test.jpg') # size 400,400

But the both following command return an error
img.paste(bg)
bg.paste(img)