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:
Drag a picture file from the Finder onto the image well (the little rectangular picture to the left of the picture’s name).
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.