Today Apple announced on their Developer Portal that screenshots for submitted apps will be locked once an app is approved. Until now a developer could change screenshots for an app (the preview images you see in the App Store that help you see what an app looks like) at any time. The new rules mean that now the screenshots for an app cannot be changed until an update for the app is submitted again.
From Apple's notice:
Updating Screenshots in iTunes Connect
Beginning January 9, app screenshots will be locked in iTunes Connect once your app has been approved. New screenshots may be uploaded when you submit a binary for an update to an existing app or a new a
