[eschwartz]

Originally Posted by theducks

It says No cover Available

I think that one is for the CONTENT of the catalog

Yup. It is only used by this method:

$ cat src/calibre/library/catalogs/epub_mobi_builder.py
...
    def generate_thumbnails(self):
        """ Generate a thumbnail cover for each book.

        Generate or retrieve a thumbnail for each cover. If nonexistent or faulty
        cover data, substitute default cover. Checks for updated default cover.
        At completion, writes self.opts.thumb_width to archive.

        Inputs:
         books_by_title (list): books to catalog

        Output:
         thumbs (list): list of referenced thumbnails
        """
...