Set the Desktop Background for all of your open Spaces in Mountain Lion

set-wallpaper.sh

read -e IMAGE;
defaults write com.apple.desktop Background "{default = {ImageFilePath='$IMAGE'; };}"
killall Dock

README.md

Instructions

1. Open Automator and create a new Service
2. Set "Service receives selected" to "image files"
3. Add a "Run Shell Script" action to the workflow, remove the default script contents, and paste the set-wallpaper.sh script into it
4. Save as "Set as Wallpaper"

The service should then be available to you when right-clicking image files. Right-clicking an image file (png, jpg, etc) in the Finder and choosing "Set as Wallpaper" will set that image as the Desktop Background image for all open Spaces. No need to quit apps or delete Spaces!

You can use an app like Services Manager to organize your Finder context menus.

Very useful - doesn't take care of the alignment settings but in itself a massive improvemnt over the dreadful process of having to go space by space :) Thank you! 👍

Right now on Mojave, script corrupts desktoppicture.db - for now only fix is to delete this file from ~/Library/Application Support/Dock/

@TheFullResolution thanks for the warning! Have you found a script that works on Mojave?