Obs.: Store desired image on
/usr/share/backgrounds/

Using gdm2setup
===============
https://launchpad.net/gdm2setup
# sudo add-apt-repository ppa:gdm2setup/gdm2setup
# sudo apt-get update
# sudo apt-get install gdm2setup
# gdm2setup

Using Ubutu-tweak
==================
http://www.ubuntugeek.com/how-do-you-change-the-boot-splash-screen-image-for-10-04-lucid-lynx.html

Change setup using gconftoll-2
==============================
sudo -u gdm gconftool-2 --set --type string --set /desktop/gnome/background/picture_filename /usr/share/gdm/themes/human/background.png

Change "/usr/share/gdm/themes/human/background.png" with
/usr/share/backgrounds/[your image]

Other form
Default GDM background on Ubuntu is /usr/share/backgrounds/warty-final-ubuntu.png
Replace wih your ones.

using gnome-appearance-properties
=================================
https://bbs.archlinux.org/viewtopic.php?pid=787809

Put gnome-appearance-properties as autostart on login screen 
# cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow/
Setup background and after ogin, remove from auto start
# rm /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop

Run using dbus
# xhost +SI:localuser:gdm
# sudo -u gdm dbus-launch gnome-appearance-properties


==========================
Not sure exactly what you mean by "login logo." If you are talking about the logo that appears with the progress bar after you log in using GDM and before your desktop is displayed, then do this:
# sudo cp /path/to/login_logo.png /usr/share/ubuntu-docs/help/images/C/ubuntuheader.png


