"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," says the short announcement aimed at Apple developers.
Changing the screenshots at a later date will obviously be possible, but this way Apple assures itself that the change will be again inspected by its own employees.
This new rule will surely go a long way towards shutting down one of the most pervasive scam methods used by bogus developers, in which they use one set of screenshots to get their apps approved by Apple, and then substitute them with others - often of extremely popular ones, or ones that exist for Android, but not yet for iOS - as soon as they are sure they will not attract immediate attention.
Unfortunately, even a few-hour-long window of availability of such apps is often enough for these crooks to earn themselves a tidy sum and run with it, as inexperienced users searching for popular apps get tricked into paying for these bogus ones.
Apple's app vetting process is not perfect, but a few more tweaks such as this one should make it considerably more effective.