Best picture I have ever seen

That is one strange picture...

To set a desktop picture, go to the Finder, pull down the application menu, and choose Preferences. From there, it's fairly obvious.

If you want to change a folder's background, open the folder, and press Command+J. This will open the view options. Choose the Global or Window tab based on whether you want to set the default, or just the particular folder, and then select one of the three radio buttons at the bottom of the dialog.