I guess I though what was most interesting here, is that iTunesConnect previously required you upload a 512x512 image.
PS - Per the link above, you want to drop the ".png" suffix from the iTunesArtwork file. It seems counter intuitive I know, but there are quite a few blog post about this issue. Apple address it directly in their documentation.
To be honest, that doc is a tad old. Xcode handles setting your app icons different now. For example, you don't need to use specific file names and edit your plist; you can just drag-and-drop your images into the Summary tab.