Set your desktop to the file created in [dropbox]/topredditdesktop/Desktop.jpg and it will change periodically

This Trigger fires every time a new post becomes one of the ten top posts in the subreddit you specify.

This Action will download a file at a given URL and add it to Dropbox at the path you specify. NOTE: 30 MB file size limit.

Set your desktop using Dropbox to the latest top /r/wallpapers post.