Authors writing books using Leanpub's workflow have occasionally reported seeing an error message when they try to upload their EPUB file to Apple, that says something like this:
You encountered this upload error because there is an image inside the EPUB that exceeds 4 million pixels. Please note this does not apply to the cover art added to the Details pane in iTunes Producer.
"The following images are too large: /OEBPS/resources/title_page.jpg"
title_page is the name we give to the file you upload for your book cover. If you upload a cover image that complies with Apple's parameters, that should work.