there a picture for it, just open that picture with Ps or w/e program that supports tga and edit that, but if u chang ethe place of the buttons or so on, u'll need to edit the script too
C:\Program Files\Z8Games\CrossFire\rez\UI\UI_Login
contains all the things u'll c on the loging screen, open up everyone of them and u'll know what u want edit.
the login box is intergrated onto the loginbg.tga