Every Mac computer comes with dozens of free images you can use for your desktop wallpaper, including photos from National Geographic. To change images, or to use images of your own, open the Desktop and Screensaver option in System Preferences. The process is essentially the same in OS X Yosemite as for previous OS X versions. Apple computers can use most image file formats as a desktop wallpaper, with the exception of animated GIFs.
Changing the Desktop Background
Click the "Apple" menu and launch "System Preferences." Click the "Desktop and Screensaver" icon.
Click the "Desktop" tab if it's not already highlighted. The large thumbnail at the top shows your current wallpaper. Directly below that thumbnail are all of the places from which you can choose a wallpaper image.
Expand any entry in the left menu by clicking the "Arrow" beside it. In addition to Apple's stock images, colors and patterns, you can look for photos in iPhoto or in folders on your Mac.
Click the "+" button if you have a folder on your computer that contains pictures you want to use. Select that folder in the navigation window that opens to add it to the menu.
Locate the photo you want and click it. At the bottom of the window, you can specify if you want images from the folder changed and how often to change them.