Addding slideshow background in gnome-control-center ?

Hello,

how do you "properly" add a slideshow to the system that will serve as a background, so that it is available to all users from the "settings" ( gnome-control-center, background section ) ?

Since when adding images in /usr/share/backgrounds ( accessible to everyone for reading ) is not enough (anymore) to make them visible / accessible in the "settings / background"?

So I have :

⋅ my dozen images in /usr/share/backgrounds/trianglify_××.jpg
⋅ the trianglify_01.xml slideshow descriptive file in /usr/share/backgrounds/contest/
⋅ and a descriptive file of the "pack" trianglify_01-wallpapers.xml in /usr/share/gnome-background-properties/

but none of those show up in "settings / background".

On the other hand, all this works well when in dconf-editor I set

/org/gnome/desktop/background/picture-uri-dark

for it to target

file:///usr/share/backgrounds/contest/trianglify_01.xml

Obviously I tried the "equivalent" locations on the user side, by putting:

⋅ my dozen images in ~/.local/share/backgrounds/
⋅ the xml* of the slideshow in ~/.local/share/backgrounds/contest/
⋅ and the xml* of the image pack in ~/.local/share/gnome-background-properties/

* paths of the various resources adapted of course accordingly.

In this case, the images appear well in "settings / background" but not the ---slideshow--- itself.

On the other hand it is still functional if I go through dconf-editor with this time

file:///home/django/.local/share/backgrounds/contest/trianglify_01.xml
as a target.

In both cases, the only slideshow offered by "parameters / background" is the one installed by default in the system ( /usr/share/backgrounds/contest/jammy.xml ).

So how do we tell gnome-control-center that other slideshow(s) background(s) are available ?

I'd settle for a user-side solution - as long as it's accessible via "settings / background".

System here is Ubuntu 22.04 quite freshly installed.

For reference ( french ) : https://forum.ubuntu-fr.org/viewtopic.php?id=2072487