OS X: Bash function to change the desktop wallpaper across all monitors and spaces. Tested on Mavericks.

gistfile1.sh

# Usage:
# wallpaper path/to/image.png

function wallpaper() {
  image=`echo $(cd $(dirname $1); pwd)/$(basename $1)`

  cd /System/Library/CoreServices

  sudo rm -f DefaultDesktop.jpg
  sudo ln -sf $image DefaultDesktop.jpg
  sudo rm ~/Library/Application\ Support/Dock/desktoppicture.db
  killall Dock

  cd -
}