The faces that are included in the users panel are really old,
the style is outdated and they are too small for modern designs
as well as hidpi screens.
https://bugzilla.gnome.org/show_bug.cgi?id=792243
The don't really make sense in GDM anymore since all the tools
that used them have been removed from there. It makes more sense
for them to live close the UI that selects and manipulates them.
At some point we should probably consider making face images themeable
so that they can be loaded in various sizes.