How to customize login sceen in Fedora 15

To customize login screen background image, run the following commands.
[changho@changhocho ~]$ su -
Password:
[root@changhocho ~]# gdm dbus-launch gnome-control-center

** (gdm-binary:2514): WARNING **: Failed to acquire org.gnome.DisplayManager
** (gdm-binary:2514): WARNING **: Could not acquire name; bailing out

By running xhost + command, disable access control.
[root@changhocho ~]# xhost +
access control disabled, clients can connect from any host
[root@changhocho ~]# gdm dbus-launch gnome-control-center

** (gdm-binary:2522): WARNING **: Failed to acquire org.gnome.DisplayManager
** (gdm-binary:2522): WARNING **: Could not acquire name; bailing out

once again let's try sudo.
[root@changhocho ~]# sudo -u gdm dbus-launch gnome-control-center
[root@changhocho ~]#

When gnome control center starts, select personal->background and then select background image want to change to.