How do I get my iPod touch to change the locked screen wallpaper?
Have you tried:


Select your iPod under the Devices section in the iTunes Source pane. 
Select the Music tab of the iPod settings. The iPod settings are displayed to the right in the main iTunes window.
Select the "Display album artwork on your iPod" checkbox.
Click Apply to save the changes. The iPod begins to update as soon as Apply is clicked.