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
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Sign up to join this communityGnome3 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"