To change your boot time grub background download a splash image and place it under /boot/grub and then edit grub.conf to add the following:
splashimage=(hd0,4)/grub/splash.xpm.gz
Note: (hd0,4) = /boot
To change gnome splash image get a splash image from GNOME-Look and place it under /usr/share/pixmaps/splash/ with the name gnome-splash.png.
To change gdm login manager get the gdm themes from GNOME-Look and use gdmsetup to install new login theme.