Mac Basics: Change your desktop

Maverick's

To track them down, hit Command-Shift-G from your desktop to open the Finder's Go to Folder window. Next, enter this path:

/Library/Screen Savers/Default Collections/

Mountain Lion

First off, the pictures that are displayed on your background are located within - 

[mactintosh HD] - users - home directory - library - desktop pictures

in order to get there you will need to hold down the option key on your keyboard and navigate to this location.  

There is a couple of tricks along the way that you'll need to know.  

1. open finder
2. choose the button at the top of the screen that gives you a three column view.  if you highlight it, it will say " show items as icons ..."
3. while holding down the option key, choose the mac mini (OS device) from the left navigation pain and then - macintosh hd (from the left third column) - library - desktop pictures
4. copy paste the pictures you would like to add or delete.  

This will allow you to retain the existing pictures and also to manipulate it as well.

after completed you can open system preferences - desktop & screen saver - to display the pictures you've added.