You can right click your desktop, select "Change Desktop Background" and choose any one of the wallpapers pre-installed, but you need to do it each time you want to change a wallpaper. What about changing a wallpaper automatically within a certain time interval? Try Wally.
Go to Applications (or Main Menu) > Ubuntu Software Center, enter wally into the box and click "Install". (Note)
Press Alt-F2, enter wally and click the "Run" button.
Right-click the Wally icon on the panel, select Settings.
Click "Folders" in the left column, then add /usr/share/backgrounds
to the Folder box and tick "Include subfolders".