I am not sure if there is an easier way, but this is what I did (i think if you are running from a CF image maybe the whole system is r/w and you can just replace it, but this is for everyone else running from livecd with profile stored on some persistent storage.)
Step1: Duplicate kerbynet folder, not much in there anyway
cp -a /cdrom/usr/local/apache2/htdocs/kerbynet /Database/
Replace /Database/kerbynet/Zeroshell.gif with your own logo same dimensions.
In postboot or preboot startup script (did think which is best, both work, post-boot maybe a visitor will catch a glimpse of the stock image)
mount -o bind /Database/kerbynet /cdrom/usr/local/apache2/htdocs/kerbynet
then of course reboot and viola.