Mac OS X bundles wallpapers with it. Whether you are on the iMac 27”, or the Macbook Pro machine, changing wallpaper on the Mac is just a click away.
Firstly, to change wallpaper on Mac OS X, go to your desktop. Now right click on the desktop using the magic mouse. This should look similar to the screenshot below:
Click on the “Change Desktop background,” this should invoke a window that shows the list of wallpaper that is bundled with the Mac OS X on your iMac and Macbook Pro.
Now you know the drill. Simply click on any of the images and the Mac desktop wallpaper will be changed instantly. Look at the left panel and you will notice that you can change folder and select different wallpaper. You can also change images every 30 minutes as shown. Go ahead and explore the wallpaper section. Have fun! 
[...] Firstly, custom wallpapers can be any image, of any resolution. But it is always better to download wallpaper of good resolution. Now, to set custom wallpaper, right click on the desktop and select “Change Desktop Background.” [...]