When your home-screen app icons slide into place, you’ll see your lock-screen wallpaper rather than a boring black background. Want to customize your wallpaper? Now, that iOS has been released for public download, you can easily set Homescreen and Lockscreen Wallpaper on your iPhone or iPod Touch running iOS 4. Unfortunately, if you have an iPhone 3G or iPod Touch 2G, the home-screen wallpaper is disabled.
How to set Wallpaper on Homescreen and Lockscreen on iOS 4
Step 1: Tap to launch Settings from the Springboard.
Step 2: Tap to select Wallpaper from the Settings Menu.
Step 3: Tap the wallpaper preview button to continue
Step 4: Tap to open Camera Roll or Wallpaper folder
Step 5: Select an image you would like to make wallpaper
Step 6: Move and scale the image, if required and then press the Set button.
Step 7: Press to choose whether you would like the image set as the wallpaper for your homescreen, lockscreen, or both.
Step 8: Press the Home button to view your newly set wallpaper.
Voila! You have now successfully set wallpaper on iPhone homescreen and lockscreen.