So far as I know, each lending library ebook is identical to the corresponding commercially available ebook.
Some ebooks come with a front cover illustration, but as you say many don't. A good example is Dune Messiah by Frank Herbert. When you look at this ebook in the store (e.g.
Kindle Store or
FictionWise) it shows up with a thumbnail of the standard paperback cover, but the ebook itself has a text-based cover page (probably a scan of the print version's title page). Since the thumbnail is what sells ebooks, the cover is judged to be unnecessary by some publishers.