These files are read by the faces(1) command which uses them to display in its window, a small image for each user's mail box message.
face files are usually created by rescaling either a gif or jpg image.
jpg -c myface.jpg | iconv -c m8 | resample -x 48 -y 48 > myface.1
If the image contains more than just your face or if the exposure is non-optimal, Russ Cox has written a tool which allows you to trim and adjust your image, though this only writes black and white images.
Mug insists for its input to be at least 100x100 pixels and the line below adds a white border around the image which is useful - giving you room to maneuver when editing.
jpg -c myface.jpg | resample -x 100 | crop -b 255 255 255 -i -30 | mug > face.1
The manual page is mug(1).
Once you have created your face, install it in /lib/face/48x48x8/f/face.1 (your directory path may vary) and edit /lib/face/.machines and /lib/face/48x48x8/.dict as appropriate (see faces(1)).
9srv.net maintains a public faces database. If you do not have a 9srv account, you can execute
cd /lib/face && con -l tcp!9srv.net!50001 | gunzip | tar -xwhich will create a README file, a "people" directory, and a "domains" directory. This will not overwrite anything in the standard distribution or changes you've made to .machinelist or any of the 48x48xn directories. The README file contains more examples for generating faces. If your email address consists of only the characters a-zA-Z0-9.@, you can add your face to the list by sending a correctly-generated face(6) file as an email attachment to add at face dot 9srv dot net. Submissions for folks with other email addresses, as well as for additional domains, will be handled manually; email a at 9srv dot net.
There is a database of faces (picons in their terminology) at Indinana University. This includes the Usenix Face saver project archives, many company logos, and other misc images. http://ftp.cs.indiana.edu/pub/faces/picons/