it work for the html index page but not find css, png background: file manager do not put it on /usr/local/www/ but in /var/db/cpelements
Why are you looking in /var/local/www/ ? That this the home dir for the GUI.
The one for the captive portal is here : /usr/local/captiveportal (you should know that nginx is running with one instance for the GUI, and another for the Captive portal and yet another one if you have the https version of the Captive portal - check out config files for nginx here : /var/etc - check files that start with nginx- )
.... and the surprise is :
In that directory you will find your "index.php" and soft links to the files you upload
with the file manager.
Like (my setup) :
[2.3.3-RELEASE][firstname.lastname@example.org]/usr/local/captiveportal: ls -al
drwxr-xr-x 2 root wheel 512 Mar 14 09:45 .
drwxr-xr-x 16 root wheel 512 Apr 13 22:00 ..
lrwxr-xr-x 1 root wheel 39 Oct 16 2015 captiveportal-bg.jpg -> /var/db/cpelements/captiveportal-bg.jpg
lrwxr-xr-x 1 root wheel 59 Jan 29 2015 captiveportal-logo_brit_hotel_relief.jpg -> /var/db/cpelements/captiveportal-logo_brit_hotel_relief.jpg
lrwxr-xr-x 1 root wheel 42 Jan 29 2015 captiveportal-style.css -> /var/db/cpelements/captiveportal-style.css
-rw-r--r-- 1 root wheel 11603 Feb 11 14:50 index.php
-rw-r--r-- 1 root wheel 10434 Feb 11 14:50 radius_accounting.inc
-rw-r--r-- 1 root wheel 6862 Feb 11 14:50 radius_authentication.inc
So, the files used by my index.php - captiveportal-bg.jpg - captiveportal-logo_brit_hotel_relief.jpg - captiveportal-style.css are perfectly well readable.
This is how it works for years now - still don't know why it doesn't for you.