The icons used by GDM can be installed globally by the sysadmin or can
be located in the user’s home directories. If installed globally
they should be in the /pixmaps/faces/
directory and the filename should be the name of the user. Face image
files should be a standard image that GTK+ can read, such as PNG or
JPEG. Face icons placed in the global face directory must be readable
to the GDM user.

If there is no global icon for the user, GDM will look in the user’s
$HOME directory for the image file. GDM will first look for the user’s
face image in ~/.face. If not found, it will try
~/.face.icon. If still not found, it will use the
value defined for “face/picture=” in the
~/.gnome2/gdm file.

If a user has no defined face image, GDM will use the
“stock_person” icon defined in the current GTK+ theme. If no
such image is defined, it will fallback to a generic face image.

Please note that loading and scaling face icons located in remote user
home directories can be a very time-consuming task. Since it not
practical to load images over NIS or NFS, GDM does not attempt to load
face images from remote home directories.