I just created a new version of my App and discovered Apple is now requires 1024x1024 iTunesArtwork. The old value was 512x512 ...
Interesting ...
You should already have a 1024x1024 artwork called iTunesArtwork@2x.png added to your app. Just upload that instead of iTunesArtwork.png.
Really? You include your iTunesArtwork in your app?
Based on:
http://developer.apple.com/library/ios/#qa/qa1686/_index.html
It is optional.