I recently received some advice on gnu.emacs.gnus on how to get this
working.  However, I believe that the specification of the Face:
header is wrong in one respect.  It states that, because of
limitations on how large a news/mail header can be and the base64
encoding that is used, the png image can be at most 740 bytes long.

As luck would have it, the 10-colour version of the 9-colour image
that I have in this image is precisely 739 bytes.  When I base64
encode it I get a header that is too long.  A suitably modified
version of the make-face shell script seems to agree with me as well.

It seems that the calculations here need to be redone.