Hi,

I was experiencing a problem where `gnus-face-from-file' was creating
invalid PNG images.  I tracked the problem down to my use of
`prefer-coding-system' to give highest priority to a multibyte coding
system.  Apparently `mm-with-unibyte-buffer' isn't sufficient when a
string inserted into it has already been decoded from its source as
multibyte.  My solution was to specify `coding-system-for-read' as
`binary' for the call to `gnus-shell-command-to-string', and I have
attached a patch to that effect (against ognus-0.18).