tiff/pbm in pyplot (ubuntu)

[Hans Georg Schaathun]

Does anyone know how to get support for tiff/pbm in pyplot on
Ubuntu (11.04)?

This used to work for me, on some system, but when I attempt
to regenerate my TIFF files on a new system, it all crashes. 
The error message is clear; TIFF and PBM are not supported, and 
the exception occurs in a call to imsave().

I know that someone will suggest to use PNG instead, converting
with ImageMagick.  Trouble is, when I load a grayscale PNG in
python I get a 3D array with three colour channels.  I need a
2D array, and I'd rather avoid the hassle of modifying all
the occurrences to accept a 3D array and check that the colour
channels are equal.

I'll be grateful for any pointers,
TIA
-- 
:-- Hans Georg