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.

1. Go to Applications (or Main Menu) > Ubuntu Software Center, enter wally into the box and click "Install". (Note)
2. Press Alt-F2, enter wally and click the "Run" button.
3. Right-click the Wally icon on the panel, select Settings.
4. Click "Folders" in the left column, then add /usr/share/backgrounds to the Folder box and tick "Include subfolders".
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In Windows, you can place a program shortcut in a startup folder for running a program automatically when the system starts. In Ubuntu, you can do the same in this way:

1. Go To System > Preferences > Startup Applications
2. Click the "Add" button.
3. Name a program (e.g. Alarm Clock).
4. Click the "Browse" button and navigate to File System > usr > bin, where programs are usually installed.
5. Select a program (e.g. alarmclock), click the "Open" button followed by the "Add" button.
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If you are in need to access hidden files then you can use these following two ways to do the same.

Show Hidden Files on your Mac

Launch the Terminal and enter these commands exactly as shown. The first command activates the ability to see the hidden files:
Code:
defaults write com.apple.Finder AppleShowAllFiles TRUE

Facebook Chat has a feature which lets its users appear invisible in the IM. So users can chat only with the person they want to chat with. While appearing invisible in Facebook Chat, users still can be online on Facebook IM network.