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 to install the wallpaper changer Variety.
Press Ctrl-Alt-T to run Terminal. Enter sudo add-apt-repository ppa:peterlevi/ppa Enter sudo apt-get update Enter sudo apt-get install variety
With this wallpaper changer, you can change wallpapers in a fixed time interval from the sources you set and run it with several other settings such as randomly applying filter effects, color and size, customizing the indicator icon and so on.