If you have a brand new MacBook Pro or Air, iMac or Mac mini you’ll likely want to personalize your background with a picture you’ve taken or a wallpaper you’ve found online. This will apply to Mac OSX 10.10 Yosemite, likely to 10.11 El Capitan as well and many previous versions of Mac OSX like Mountain Lion or Snow Leopard.
Step 1) In the upper left corner click on the OSX Apple menu.
Step 2) In the Apple menu point your mouse to the section called System Preferences.
Step 3) Click on the Desktop & Screen Saver icon.
Step 4) Make sure the label Desktop is selected in the upper middle section of the application screen.
Step 5) On the lower left corner of the Desktop & Screen Saver app screen click on the little + to open up the Finder.
Step 6) Navigate to the downloads folder on the left side panel in the Finder (or the folder you’ve downloaded your image to).
Step 7) Scroll to the folder or location of the image you want to use as the new background.
Step 8) Once you have selected the image click on the Choose button in the lower right corner of the osx app screen.
Step 9) On the your keyboard hold down the Command-M keys to minimize the current window or Command-Option-M keys to close all current app windows so only your desktop screen is showing.
Step 10) Enjoy your new OSX background!