If you don't want a sharepoint mounted, and you don't want a package to deploy it, your options are kind of limited. There aren't that many good ways to do this.
Yet, there are a few options.
There are things like scp, but you would need to embed a username and password into a script to connect to a remote system or share to pull down the file, so that's not a great idea.
You can host the wallpaper on a webpage if you have something internal or can host it on a permanent external site, and use curl to pull it down.
Lastly, there are ways of embedding zip files into a script that can be used to deploy a file or package, so its really all just text, and then it can be unzipped into place. That's somewhat more complicated to do, but its an option.
