Hey, oha. I don't remember it precisely anymore since the last time i modded and hacked symbian was almost 2 years ago.
But this will give you the idea. Search the latest method to hack N86. Download the open4all, domainsrv and c2z4bin patches for ROM patcher.

Add a custom splashscreen.mif into C: drive which the system will read on boot instead of the one at Z: (ROM) drive. The boot image fills the entire screen.

You could also change the shutdown screen except it's easier. Anyway, you better register at some symbian forum like http://www.symbian-freak.com/forum