Changing your Mac’s Desktop picture is easy: just click the Apple menu and choose System Preferences. Select Desktop & Screen Saver and click the Desktop tab. Then select Choose Folder and navigate to the folder containing the picture you desire.
You can also create a custom desktop slideshow by putting the images you’d like to use inside a new folder, navigating to it as described above, and selecting one of the images. Before you close the Desktop & Screen Saver window, select the Change Picture checkbox. Using the pull-down menu, specify how quickly you’d like the images to change and whether you’d like the images to cycle in random order.
To keep things tidy, you may want to store your new custom picture folder in the same location as your default Mac OS X desktops. They reside in your root-level Library folder, in the folder called Desktop Pictures.