Heh, you love customizing your PC to the core or what? Let it look like Linux for the OS's sake!
Anyway, here's what you ordered for
Install (If not present):
Code:
sudo apt-get install gnome-splashscreen-manager
Run (Through Alt+F2 or Terminal):
Code:
gnome-splashscreen-manager
[Edit: Also found via
System > Preferences > Splash Screen]
Click
Install, choose the image file (PNG or JPG) and it must appear in the window thus. Click
Activate, re-login to see it in effect.
One small tip:
Code:
sudo apt-get install gnome-art
System > Preferences > Gnome Art
That's gonna be your favorite tool hence.