Sideloaded NOOK Book Covers: Why No Thumbnails?

A few weeks ago when I was preparing my story "Safarka's Butterflies" in ebook form, I stumbled across a problem. I was opening the ebook in Kindle, iBooks, Kobo, and NOOK to do some quality control. The cover appeared wonderfully in the Kindle, iBooks, and Kobo but not in NOOK--even though iBooks, Kobo, and NOOK used the same ePub file. Well, after a bit of searching, I found an answer on the B&N support pages, but it took a bit of digging. I offer it here.

Open the ePub in Calibre (a free ebook manager).Ensure the cover appears in Calibre.Close Calibre.Open the exact same ePub file in Sigil (a free ePub editor).In Sigil, click Save (you don't need to change anything.Reopen Calibre.Select the book in Calibre and click Convert Books. This opens the Convert dialog box.Ensure that the Input Format and Output Format say EPUB.Click OK. The file is converted.Now when you sideload to your NOOK, you'll have a cover.