login is handled by GDM or KDM for gnome/KDE respectively.
For GDM you can either configure it from the login page (actions>configure login manager) or from the gnome control centre thingie.
For KDM you can configure it from the KDE control centre.
I know that GDM takes the tar.gz and installs it, don't know if you have to unpack it for KDM.
For splash screens do you mean the DE startup or the bootup from GRUB? KDE splash (when kde starts) can be configured/installed from control centre again, no idea for gnome, sorry.