How to Add Pictures to Your Mountain Lion Desktop
When customizing your Mac OS X Mountain Lion workspace, the first thing you probably want to do is to place a personal picture of your family or family pet on the desktop.
Here’s how you can change your Desktop picture if you care to:
From the Desktop, choose Apple→System Preferences.
Or Control-click the Desktop itself and choose Change Desktop Background from the contextual menu, and skip to Step 3.
The System Preferences window appears.
Click the Desktop & Screen Saver icon.
When the Desktop & Screen Saver Preferences pane appears, click the Desktop tab if it’s not selected already.
Click a folder in the column on the left, and then click a picture in the area on the right.
You have at least three other ways to change your Desktop picture:
Choose the Pictures Folder in the list of folders on the left side of the Desktop & Screen Saver System Preference pane, and then choose a folder by using the standard Open File dialog. That folder then appears in the list; you can use any picture files it contains for your Desktop picture.
Click one of the iPhoto Albums items in the column on the left side of the Desktop & Screen Saver System Preferences pane.