Apple sent out a notice last week to developers telling them that they should submit app updates that made their apps iOS 6 compliant as soon as possible. Strangely, the focus was totally on iOS 6 rather than the new iPhone, which has a new sized display.
Because of this change, Apple wants screenshots at variations of the new size. Here are the screenshot sizes Apple says it will accept:
640 x 1136 (portrait)Apple warns that if a developer submits an app that takes advantage of the new iPhone 5 screen size, but does not submit properly sized screenshots the the status of your app will change to Missing Screenshot.
640 x 1096 (portrait)
1136 x 640 (landscape)
1136 x 600 (landscape)