The former way to change wallpaper in Gnome2 consists in use gconftool-2, but this tool has no effect in Gnome3.
gconftool-2 --type=string --set /desktop/gnome/background/picture_filename /home/user/background.jpg
Gnome3 uses dconf instead of gconf to background and wallpaper issues. The tool to modify dconf is gsettings and it can be done by the following way:
gsettings set org.gnome.desktop.background picture-uri "file:///home/user/background.jpg"
