True, thanks sud0nick ! Symbolic links are a good friend.
for those who have the same issue or question:
ln -s /usb/nodogsplash/htdocs/images /etc/nodogsplash/htdocs/images
and
ln -s /usb/nodogsplash/htdocs/splash.html /etc/nodogsplash/htdocs/splash.html
note:
neither splash.html nor images folder should exist in /etc/nodogsplash/htdocs before launching the above commands.
Another interesting link: