Since the discussion started here, I'll respond here.
I've seen
your issue crutledge, and I've examined the two files. There's absolutely nothing wrong with the epub file Sigil produced. You marked the cover image as the cover, and Sigil added a special meta tag identifying the cover to the OPF. RS's like the iPad will be able to load this cover and show it on the bookshelf.
Again, the epub itself is
perfectly valid and also quite sensible.
Calibre then loads this epub file and invents a "titlepage.xhtml" file
out of thin air that wraps your cover image in an SVG and then adds it to the front of your book. My first reaction would be that this is a bug in Calibre, but since Kovid has seen the files, I guess it's a feature. I'm guessing it's because Kovid wants your epub file to also have a special XHTML page for the cover that is also present in the guide section of the OPF. This is not necessary at all, and I'm
strongly against a converter making these kinds of decisions about the "preferred" structure of EPUB files.*
An irrefutable fact of the matter is that your original epub file was valid and completely sensible.
Kovid's comment that "There needs to be a guide element in the OPF file" is patently false (sorry Kovid but it's true, and you know it). There doesn't need to be any such thing. You
could add it, but
nothing says you should.
If you want to add this information to your book, you could do so by right clicking the XHTML file containing the cover image in the original file and going to Add Semantics -> Cover. This would prevent Calibre from inventing new content files.
*This "feature"
really pisses me off. Inventing new files out of the blue and casually inserting them into the book? C'mon. I can understand the motivation and the good intentions behind it, but this is a damn good example of those intentions paving a road...