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 app.
The change is being made in an attempt to thwart a popular trick scammers use in which they try to trick users into buying fake apps -- usually games -- that steal screenshots from popular apps in an attempt to make the user think they are downloading the legitimate app. Under the old method, a scammer could upload an app with a screenshot that represented the app as it was and then switch out the screenshot with fake ones after the app had been approved.
Usually these fake apps are removed quickly once reported and customers are refunded, but the new rules are an extra measure that could help save customers a lot of frustration and should also help protect legit developers' intellectual property.