The older way of doing this, with gconftool-2 doesn’t seem to work anymore in GNOME 3 – used to be something like: gconftool-2 –type string –set /desktop/gnome/background/picture_filename “/full/path/to/file.png”.
However, there is still possible to change the background image, by using the gsettings tool instead. You will need the libglib2.0-bin package, which is probably already installed on your system. To change the background, use a command like the following:

