Windows 8 Developer Preview was launched last week, its loaded with plenty of new features, notably the lock screen. This screen is a part of Microsoft's Metro-style User Interface. Here you can customize the Windows 8 lock-screen by changing the default picture to your favorite outdoor photo.
Below is the default lock-screen image:
To change this picture:
1. just log into your account
2. Click on your user name on the top right of the window
3. Click on 'Change User tile'
4. Under 'Personalize' option, click on 'Lock Screen'
5. Now the screen will be listed with number of pictures
6. Click on the picture that you wish to display to your lock screen.
This video will help you to find out the options to Change lock screen picture, Restart or shutdown your Windows 8 computer.