Using exact (case sensitive) matching?
Not a bug.
cover.jpg is not equal to Cover.jpg
Code:
<div><div><img src="cover.jpeg" alt="" style="width:100%;"/></div>
But BookBrowser shows the file is actually called "Cover.jpg"
All urls use case sensitive paths. This is the spec. Are you on Windows with a case insensitive file system?
Epubcheck should easily detect this case and report it.