Oort Gnus can use a Face header instead of an X-Face header (see GnusXFace). The Face header uses a mimencoded color PNG file. For more info, see the Gnus page:
Here are two ways of getting a Face header. This one will use a JPG file, pipe it through tons of utilities, and reduce colors until it is small enough.
(gnus-face-from-file "~/pics/alex-face.jpg")
Note that the utilities needed are listed on the link above, and the corresponding Debian packages are (in sid) : libjpeg-progs and netpbm. The function should return a Face. If not, one utility is missing.
If you have a hand-crafted PNG file, 48x48 and smaller than or equal to 726 bytes, use the following:
(gnus-convert-png-to-face "~/pics/alex.png")
And here is an example of how to use it using PostingStyles:
(setq gnus-posting-styles `((".*" (address "alex@gnu.org") ("Face" "iVBORw0KGgoAAAANSUhEUgAAADAAAAAwBAMAAAClLOS0AAAAG1BMVEX///89Pjl6hX25yeD/ 373gtJdgTEW6a14sduMzR7J4AAAAAXRSTlMAQObYZgAAAeJJREFUeNp1lMGSmzAMhvMKolOaa5wB cg3p5gGy8va8xCZn0658LlPg3Bw6PHZlArbZ2XUOJHz+f1mylM3pk7UJX8t/zf4DUOYAlJJnHuyA ARgCIVagNLCsZA0grMkuBrQQ5/aRAtII7GIA+f4TAM11AWYN4OcMntwPiuh+E2I3Q0GUphNNF3Di /UU2DD29UzxB2rZZpat+8iQKVkmRaV7VtJ988BPYttMaUebrU5XQuP0KsTJrkAoHkEH+DnQzmCVf PJiskL3kbq0goW996zR1DLhWqb0RtBniS5ygK2Jy6/lsCl8M57FU1135V6xTOyi8pDYDfx85mF+o h46DXKj7QQsoBdEflC5DvGRKZl4hsp5fToAfOlytEAPOCn6qAIqiUVJ2DJRTBnAuLGd+69Alj/Ia QG8s6dubcgJVBkVJ9I3rOExBpIgURG9aYuWKJesmKE5Edpjy49B5E81Hbsg+7hDxdxKBM1vNV/Vs ksiqtFxA5cD3V4itarKt5sPK51cDIjouWO4rKZE7y8A9gCM0mVaSo5ir2R68FTc194l7XycCxoNX HF23Z7oSBYkdHILVY4yaPDdJ9Xc7enD0Y2mSHMZxvM/AjxIBg/soZnD2fwrcXrBlhXiAZYweQzje +XP6D3vk9qXUjAFlAAAAAElFTkSuQmCC")) ("gnus.ding" ("Mail-Followup-To" "ding@gnus.org")) ("emacs.sources" ("Followup-To" "gnu.emacs.help"))))