dconf write a new key : dconf write KEY VALUE
to write string value foo, key must be "'foo'"
set wallpaper :
dconf write /org/gnome/desktop/background/picture-uri "'file:///home/user/path/to/image.jpg'"
read current:
dconf read /org/gnome/desktop/background/picture-uri
result is
'file:///home/user/path/to/image.jpg'
need to parse !