Do you have multiple desktops setup on your Mac and want to change the background image of all of them at once? Since OS X seems to only allow you to change them one by one I have developed a Preference Pane to make this task a breeze. First you will need to download the Preference Pane from here then open it and install it.
Now in System Settings you will need to click on the wALLpaper Preference Pane.
Once you are in this new Preference Pane you will see your current desktop background image and a button that says Change Wallpaper…, click this button.
Now you will be presented with a file dialog which defaults to the folder where all of the Apple desktop wallpapers are saved. Choose the wallpaper you want then click Open.
The wallpaper preview in the Preference Pane will update to the new wallpaper you selected and your desktop backgrounds should also be updated on all desktops.
I hope this simple tool is useful to you and makes this task as simple as it should be. Also if you are a developer and would like to see the source code behind the Preference Pane you can find it here.