It seems gconftool-2 from comment #2 is being deprecated. Here's how to fix this in 13.10:
dconf write /org/gnome/desktop/background/picture-options "'spanned'"
It seems gconftool-2 from comment #2 is being deprecated. Here's how to fix this in 13.10:
dconf write /org/gnome/ desktop/ background/ picture- options "'spanned'"