Add wallpaper

If you are using Gnome, right-click on the desktop and select "change desktop background". There you can add your own wallpapers if you want or change to a different existing one.

The default system wallpapers can be found in /usr/share/backgrounds

I see. So, the background change every day. How can i alternate the frequency?

I have a new question. How can i change my wallpapers randomly without download a software? When you choose a new background is possible use the default wallpapers randomly. I want do it in my wallpapers.

Searching "wallpaper" in the Synaptic package managers returns quite some solutions: Wallch, Cortina, DesktopNova, etc.

https://help.ubuntu.com/community/SlideshowWallpapers
http://www.webupd8.org/2011/12/create-wallpaper-slideshow-with-xml.html

Thanks to everybody. And thank you icarolongo you solved my question. But i thought was more easy... Well, i going to install a virtual machine and try this solution.

One alternative is to use feh.
$ apt-get install feh #Install feh

Create this file, name it whatever.

#!/bin/bash

delay=10           #Timeout between image switch (seconds)
directory=~/Images #Directory with files

while true; do
    #Add '--recursive' in order to include sub folders too.
    feh --bg-fill --randomize $directory
    sleep $delay
done

$ chmod +x FILEPATH #make it executable
$ cp FILEPATH /usr/bin #refer to the file only by it's name not the full path