Thanks , it is ok on ubuntu, but it seems fedora don't support that configuration.
Today I download a gdm login window theme package. Results by google searching shows it is very easy on ubuntu by entering System-> Administration -> login window. But I don't find that way on fedora now.
The theme package I download is named GDM-BrasillinuxEye.tar.gz . I download it from official web
http://art.gnome.org/backgrounds/