Tried to submit an update the other day, and strangely as below, got this "Missing Screenshot" as shown below.
At the details view a slightly more helpful popup message was shown.
It took me a while to find the problem. As shown below, the middle part, we now have a new section for "iPhone 5 and iPod touch (5th ten) Screenshots". The only difference is the new screen size compared to previous version of iPhone/iPod.
Too lazy to design new ones, so I just use Inkscape to increase the width of the existing iPhone/iPod ones a bit, submit the new screenshots, and that fixed the problem. So for those who hasn't or is going to submit new app or updates, make sure you have 3 sets of screenshots ready!