Apple has announced on its Developer Portal (http://tinyurl.com/bzhcn5t) that screenshots added to app descriptions will be locked in place once an app has been approved.
“Beginning January 9, app screenshots will be locked in iTunes Connect once your app has been approved,” Apple says. “New screenshots may be uploaded when you submit a binary for an update to an existing app or a new app.”
For more information on capturing and using screenshots, read the Xcode User Guide (http://tinyurl.com/agpc7hs).