You can change the wallpaper by selecting a picture that you like. If it doesn't quite fit, you can choose how to crop it. The lock screen will show the new wallpaper and information that might be important to you, like the time and date, or missed phone calls and new voice mail messages, text messages, alarms, appointments, or emails.

When you're listening to music, you can also have your phone show the artist who's currently playing on your lock screen.

To change the wallpaper

  • On Start, flick left to go to the App list, and then tap Settings > Lock + wallpaper.
  • Tap Change wallpaper.
  • Navigate to the picture that you want to use as your wallpaper. If the picture doesn't fit in the frame, pan the picture until you have it where
    you like it, and then tap Done.

    Tip

    If you turn your phone off and then back on, you can see the new wallpaper right away.

To show artist art on the lock screen

  • On Start, flick left to go to the App list, and then tap Settings > Lock + wallpaper.
  • Turn on Show artist art when playing music.

For more tips, go to: http://www.microsoft.com/windowsphone/en-us/howto/wp7/basics/lock-screens-faq.aspx