Cinnamon: system wide wallpapers for regular Joes

Pretty much like I did on this old post, during the Golden GNOME 2 days...
Karmic: System wide wallpaper for regular joes

So, start by copying your custom wallpaper to the directory:

/usr/share/backgrounds/linuxmint-maya-extra/

And then, edit the text file:

/usr/share/gnome-background-properties/linuxmint-maya-extra.xml

After that, when you open the "Change Desktop Background" your newly added wallpaper will be listed along Cinnamon's default ones.