Have a section on the map that has the login screen graphics, then when the player logs in, have them start at that location, but without anything about them really defined.

Alternatively, you could just have a login HUD put on the player's screen with objects you could click to do choose new, load, etc.

Login/Loading screens can be a little tricky.

The following should help. The first is an example of what I believe you're looking for, the second is a demo of Deadron's Character Handling system.
http://developer.byond.com/hub/Griswald/CharacterHandling
http://developer.byond.com/hub/Deadron/CharacterSaving