Type this in a terminal (change 4 to the number of desired workspaces):
gsettings set org.gnome.desktop.wm.preferences num-workspaces 4 gsettings set org.gnome.shell.overrides dynamic-workspaces false
Note: I have only checked this with GNOME 3.4 — YMMV

# put in your bashrc
chwall () {
# change img folder according to their system
gsettings set org.gnome.desktop.background picture-uri file://`ls ~/Imagens/wallpapers/* | shuf | tail -1`
}
# to see more readable version visit
http://pastebin.com/fViuFhBw
great!
thanks!