Customizing the computer icon is most easily achieved using Ubuntu Tweak, but you can also configure it manually by copying your selected icon to the /usr/share/icons/Loginicons/apps/64 directory (for Ubuntu 10.04 & 10.10). First, you'll need to rename the existing icon. Open a terminal and type:
sudo mv /usr/share/icons/Loginicons/apps/64/computer.svg /usr/share/icons/Loginicons/apps/64/computerBAK.svg
Next, copy your new icon to the directory making sure that it is renamed computer.svg:
sudo cp /icon_location/icon_name.jpg /usr/share/icons/Loginicons/apps/64/computer.svg
Now, let's add a bespoke login greeting or instruction to our login dialog:
sudo -u gdm gconftool-2 --type string --set /apps/gdm/simple-greeter/banner_message_text "text"
Where text is the text that you want to appear on your login dialog. Now we need to invoke the message at start up:
sudo -u gdm gconftool-2--type bool --set /apps/gdm/simple-greeter/banner_message_enable true
Your bespoke message will now appear next time you boot your system.
Adding button icons is just as easy. The icons will follow the system's default theme (in Ubuntu 10.10, the system theme is Loginicons). In a terminal:
sudo -u gdm gconftool-2 --type bool --set /desktop/gnome/interface/buttons_have_icons true
There you have it; a few minor tweaks and you have a customized boot sequence.
Sources & References:
Towheed Mohammed: A Comprehensive Guide to Customizing the Graphics in Grub, Usplash, Xsplash and GDM
Artwork Backgrounds & Icons - Ubuntu Tribal Pack by thewer Gnome-Look.org Download