Information about the Face header:

A function to create a Face header:

 (defun insert-face-header (file)
   "Insert a Face header at point."
   (interactive "*fPNG File: ")
   (let ((coding-system-for-read 'binary)
         (coding-system-for-write 'binary)
       (insert-file-contents file)
       (goto-char (point-min))
       (or (looking-at "\x89\x50\x4e\x47\x0d\x0a\x1a\x0a")
           (error "%s does't look like a PNG file"
                  (file-name-nondirectory file)))
       (base64-encode-region (point-min) (point-max) t)
       (setq face (buffer-string)))
     (insert "Face: " face "\n")
     (forward-line -1)
     (while (and (= 77 (move-to-column 77))
                 (not (eolp)))
       (insert "\n "))
     (forward-line 1)))

If you want to create a face PNG from shell, you’ll need something like this (change the ppmquant till you get the right size):

 apt-get install netpbm libjpeg-progs / libjpeg-progs-mmx
 djpeg foo.jpg | ppmnorm | pnmscale -width 48 -height 48 | ppmquant 7 | pnmtopng > foo-face.png

Note: Gnus has the function gnus-face-from-file, you should use that instead.

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