What's going on is this:
This is a fanfiction, so it's target audience already knows about it and perhaps just wants a more portable format to read. I had downloaded other fanfictions from Archive of Our Own to basically 'archive' on my Kindle for when I wanted to re-read them, but the auto-generated .mobi file was horrendously formatted. I thus decided to "roll my own". Now, this particular fanfiction is being reworked (due to author realizing serious plot holes), so I'm formatting updating mobi/epub versions with each new chapter (we're already over 25k words at 4 chapters in).
In the Chapters 1-3 version, the kindle mobi worked just fine without a cover. Chapter 4 actually includes an image during the story, so I
it. But now when I convert the html version, it includes that image at the top of the book as though trying to use it as a cover image AS WELL AS in its proper location. If I click the "remove first image", Calibre removes the 'cover' usage, but it also removes the intended usage.
But I guess I'll just throw something together for a proper cover image, and if the author has something else she'd rather use, I'll replace it.