Main Menu>System Settings>Login Screen

...at least that's where I see the option while running GNOME.

Check out your /etc/motd file. It may be in other locations for other systems, but that's the default location for the message of the day. HTH

It depends on how users are logging in.

If it's through telnet, you would edit /etc/issue.net. Otherwise, it's dependant upon each particular service. /etc/motd would be displayed for users after they've logged on.

Interestingly enough, if your terminal supports it, you can do some neat things with /etc/issue.net and its local equivalent, /etc/issue . If you fancy yourself an ASCII artist, and you know escape codes to generate colored text, you can draw and color neat little ASCII art pictures. Once, I drew an orange castle, complete with a blue sky and yellow blinking stars, and I used it for a machine I named "fortress"