An optional message header in Usenet newsgroups and E-Mail.
X-Face is sort of a graphical .signature, taking approximately 3 lines at most. It has a compressed black-and-white bitmap, and as name implies, it usually used to hold user's face photo, but can be used to hold any 48x48 pixel bitmap.
The headers are generated and decoded using "compface" library. Many graphical news readers - including exmh and more popularly Gnus - support X-Face out of box.
See also: X-Headers