Plain old GNOME 2 had a panel applet which displayed your user long name. It had a menu option under which you could set an image to be a login avatar, and it appeared in the gdm display manager (aka login screen). Now I'm using F17 with MATE and I don't see such an option anywhere. I believe there's some package from the MATE repository I need to have to have this functionality, but which one?
are you using gdm 3.x ? if yes, then the account properties are managed trough the dbus service org.freedesktop.Accounts.
You can install d-feet, which is a python+gtk browser of the dbus, conect to system bus, search for the org.freedesktop.Accounts service, then the object path "/org/freedesktop/Accounts/UserXXXXX" (with XXXX your uid), interface "org.freedesktop.Accounts.User", method SetIconFile and give a path name (in "quotes") to an icon.
or with the command line tool:
dbus-send --system --dest=org.freedesktop.Accounts \ --type=method_call --print-reply \ /org/freedesktop/Accounts/User$(id -u) \ org.freedesktop.Accounts.User.SetIconFile \ string:"/some/path/to/tour/file.png"
also, MATE people could backport the old gnome2 applet, and change it to use that dbus service (hint: copy files um-user.c and um-user.h from the gnome3 control-center and then all needed functions will be easily available)