According to this article you can submit a single screenshot size and have iTunes connect generate screenshots for all other screen sized.
I tried uploading a single set of screenshots under the "Submit for review" screen" but unfortunately I was unable to make it work..
What am I missing?
Itunesconnect doesn't generate screenshot for other size rather it uses one size screenshot for all iphones and one size for all ipads that "you" upload.
What you need to do is
Similarly if you are making an iPad app then
That's it you are done!