I was stumped by how to quickly establish a picture as a Desktop background image. After some thought, I have created an interesting solution. First, launch Automator, in your Applications folder. Once it's running, choose File -> New (if necessary) to give yourself a blank working slate.
Then follow these steps to create your new Desktop picture changing application:
- Choose File -> New
- In the Library column, choose Applications -> Finder.
- In the Action column, double-click on the Get Specified Finder Items entry.
- In the same Action column, double-click on the Set the Desktop Picture item.
- Now choose File -> Save As. Here we type the name, for example Picture to Desktop, and below in the File Format drop-down list, change Workflow to Application.
- Specify any place to save file. For example, your Home directory.
- Open your Home directory. It is possible (with the Get Info window for the new file) to paste a new icon onto the application, in case you don't want the default Automator icon.
- Don't close your Home directory, and then drag your new application to the Dock.
The program reacts to files which are not the images as follows: it deletes existing wallpapers, and does not put anything in exchange -- simply a screen of continuous color. It is possibly to get rid of this issue, if between Get Specified Finder Items and Set the Desktop Picture you insert the Filter Finder Items Action, and list the corresponding filters for image files.
[robg adds: In my testing, the results were somewhat inconsistent -- sometimes this script worked, sometimes it didn't. I tried using various methods of getting the Finder selection, but everything I tried had the same issue. I'm not sure if it's something particular to my setup, or just a not-quite-there implementation of setting the desktop picture via Automator.]

