Recently, I wanted to find out where the file I was currently using as GNOME/Unity Ubuntu desktop wallpaper - as it had been quite some time since I've selected it, and wanted to move somethings around my system, without breaking stuff, or accidentally deleting the background image I was using.
First, I tried the GUI System-Settings tab, only to find that it didn't show the current file used, at all!
A bit of research online, and I found an interesting difference between Ubuntu versions which might be helpful for others to know:
Up until Ubuntu 10.10 you would ask gconf on command-line to echo the current value:
gconftool-2 -g /desktop/gnome/background/picture_filename
At least, this is what you can read here: http://ubuntuforums.org/showthread.php?t=1372996
But, since Ubuntu 11.04 it is
gsettings get org.gnome.desktop.background picture-uri
This is handy to know. Now, in order to change this value, to change the current background desktop wallpaper, simply replace get with set and you can update/change the gsettings value.