If you do nor override the Start, Devices open the first file in the book. For EPUB2, that is the cover.
If you want to use the Editor , 2 steps
you add the Image file, right-click the filename: mark as cover.
You create a 'cover page' , place the image and any text. Then right-click that filename; mark as cover
Both items must be marked.
Why not Use Calibre Metadata Editor?
Simply fill out the form, drop the prepared cover (Generate Cover tool). save (OK)
Then
Polish this into your book (that must be in the Library)
Presto. A cover (titlepage.xhtml and Jacket.xhtml )