Encouraging news, thanks, since for the moment this will be for the Kindle store. Since I hope to later submit it to other stores in ePub format, sounds like I'll have to resolve this then.
I have these two lines in the content.opf, so I guess I have this covered for the mobi version:
<item href="Images/cover.jpg" id="cover.jpg" media-type="image/jpeg" />
<item href="Images/cover_fmt.jpeg" id="cover_fmt.jpeg" media-type="image/jpeg" />
Thanks to all who have responded in this thread, and beyond that, to the many helpful members who have replied elsewhere to my queries. What a great forum!