Which files do you mean exactly by boot and splash images? If you mean the NOKIA logo shown at startup and the startup animation, those are not coming from the theme. The very first NOKIA logo comes from the boot loader (NOLO) and can't really be changed. Then the same logo gets displayed by MProgressIndicator and that stuff is under /usr/share/MProgressIndicator. It can only be changed the ugly way by modifying and replacing some files there, not by theming.