[Image-SIG] error in ImageChops.logical_xor ???

[john taylor]

hi all, 

i want to compare two images, they are of the same
mode ('RGB'), same format ('BMP'), same size, etc. 

i am using ImageChops.logical_xor(img1, img2), but it
gives me this error:

Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
  File
"C:\Python24\Lib\site-packages\PIL\ImageChops.py",
line 262, in logical_xor
    return image1._new(image1.im.chop_xor(image2.im))
ValueError: image has wrong mode

i've also tried calling img1.im.chop_xor(img2.im), and
not surprisingly i get:

Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
ValueError: image has wrong mode

anybody can help on this? and which is the fastest way
to compare two images, if i only want to know if they
are identical or not?

thanks a lot!
john