Ich würde das versuchen:
$ gconftool-2 --type list --list-type string --set /apps/compiz/plugins/wallpaper/screen0/options/images "[file:/path/to/first.png:100,file:/path/to/second.png:100]"
Es setzt erfolgreich diesen Schlüssel / Wert für mich:
$ gconftool-2 -R /apps/compiz/plugins/wallpaper /apps/compiz/plugins/wallpaper/screen0: /apps/compiz/plugins/wallpaper/screen0/options: bg_color2 = [] bg_image_pos = [] bg_fill_type = [] images = [file:/path/to/first.png:100,file:/path/to/second.png:100] bg_image = [] bg_color1 = []