Original report for this ticket is dealt with, closing as not a bug.

This sounds like a bug. I think the per-gallery should supersede the 
user pref. The user-pref should really only be used to provide a 
default for this value. Can you please create a new bug report for 
this piece?
Not sure. Either way, this would be a non-ansel issue, so please 
create a separate ticket in the base horde queue.

Thanks.

Thanks for narrowing it down so tightly ! Very easy to track it, I got 
it to echo each case in the switch and the case 'all' got the hands up !

When I saw this, I got it. There is two sets of permissions for 
download, one in the property of the gallery and one in the 
preferences of the users. The preference of the users supersede the 
property of the gallery. Later this week I'll try to put a "if" to 
grey out the property setting if the user preference is at "anyone" 
since this gallery setting become useless in this case since it's 
superseded and while greying it out i'll try to put a explication why 
it's greyed out on it's side.

Still on the Slug subject, while viewing a picture as a guest, I get 
after a while a session expired !? What session, I'm a guest not 
logged in ?! The Horde session settings are all defaults and the only 
timer for expiration is a 72000 sec, which is a lot longer than what 
I'm seeing here for guest expiration.

You need to add it to the 'Ansel' application, yes.
I cannot reproduce this, though I see it on your link. You will have 
to track this down yourself. Look in Ansel_Gallery:canDownload() to 
see which case is returning true.

You were right. I had to turn on rewrite module as well. Didn't though 
about that one.

Continuing on the slugs, I've setup general permission in Horde for 
guest to show and read ( only way possible I found for a guest to see 
the gallery was to add this general permission in Horde, is that right 
? ), I've also set-up preference in Ansel as only authenticated user 
could download original picture. In the Gallery preference are only at 
show and read for guest.

While viewing the gallery thru the slug as a guest, I get the link on 
top of the picture to download the original picture. ( btw you can try 
with the link it's working now )

I've also tried that with the setting at "Users with edit permission" 
and the download link is still available

Sounds like you didn't turn on "pretty" urls in the Horde config.

Ansel is configured to work with a files on the local system and with 
VFS direct source that seems to be working correctly in horde. I can 
also access file directly by typing the url of it.

I configured a slug named "test" and when I access

http://mail.spagi.net/ansel/gallery/test ( with or without the 
trailing / I tried both ) I get a 404 not found.

Apache log confirm the 404

192.168.0.1 - - [17/Sep/2014:21:36:52 -0400] "GET 
/ansel/gallery/walkingtherockies/ HTTP/1.1" 404 525 "-" "Mozilla/5.0 
(Windows NT 6.1; WOW64; rv:32.0) Gecko/20100101 Firefox/32.0"

Horde is installed in /horde/ on Ubuntu and apache 2.4 configuration 
is straight forward as follow :

<VirtualHost *:80>
         ServerAdmin /*SNIP*/
         DocumentRoot /horde/
         <Directory /horde>
             Require all granted
         </Directory>
         ErrorLog ${APACHE_LOG_DIR}/error.log
         CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>

Something I'm missing or slug really don't work ?