Don't worry! It can be fixed... and tested! Just create an image (jpg or png) and copy it to
/usr/share/pixmaps/splash/. Now launch a terminal window and type
gconf-editor. This will launch the Gnome Configuration Editor.
Navigate to apps >> gnome-session >> options and change the
splash_imageto your image name. That's it!
Now to test it without logging out. First install Xnest:
sudo apt-get install xnest. Now go to a terminal window and type
This will launch another login session inside a window, so you don't have to keep logging in and out to test your new splash screen!