Newer emacsen support X-Face headers (see GnusXFace and XFace) and the newer Gnus versions support Face headers (see GnusFace and XFace), as do newer MH-E versions.
Emacs 20 cannot do X-Faces (as used in Newsreaders, for example – see GnusXFace). Debian has a package to do this: They convert the X-face into a StippleAttribute and use that instead. The same trick is used in the Gnus splash screen.
Anyway, if you are interested in the lisp files, check here:
Note that XEmacs and Emacs 21 seem not to need this. See CompFace for more on creating X-Faces, and BbdbFaces for more on storing them in the Big Brother Database.
(Thanks to Lo-lan-do on #emacs.)