How can I make background image changed as time goes?

Variety is a tool which can do this (and much more) pretty easy!

since it is not in the default repositories you can install Variety via terminal (CTRL+ALT+T) by the following commands:

sudo add-apt-repository ppa:peterlevi/ppa
sudo apt-get update
sudo apt-get install variety

1. Download the script from HERE.

2. Create a new folder in your home partition, let's call it "earthwallpaper" and put the script you just downloaded into this folder.

3. Open a terminal, navigate to the folder you created at step 2 and make the script executable:

   ~/earthwallpaper
   chmod +x changer.sh
   

4. Go to System > Preferences > Startup Applications and select "Add", in the name field enter "Earth Wallpaper" and in the command field, enter:

   /path/to/changer.sh
   

Changing /path/to with the path to were you copied the changer.sh file you downloaded at step 1.

Now, each time you start your computer, the script which updates your wallpaper will automatically run.

5. Log out and log back in. Running the script creates a new wallpaper called world_sunlight_Wallpaper.jpg in your ~/.gnome2 folder. We need to set this as your wallpaper. This picture is constantly updated by the script, thus making the image dynamic. To do this, open Nautilus, toggle the text-based address bar and type:

   ~/.gnome2
   

   Which will open the hidden .gnome2 folder from your home directory. Now, on your desktop, right click and select "Change Desktop Background" which will open the Appearance dialog. Having both the Appearance dialog opened and the ~/.gnome2 folder, drag and drop the world_sunlight_Wallpaper.jpg file from Nautilus, into the Appearance dialog (Background tab). That's it. Now you just have to keep the terminal opened so that the script will continue to update the world_sunlight_Wallpaper.jpg wallpaper in real-time.

References:

1. webpd8.org
2. Stack Overflow