I would love to get to the bottom of the "missing thumbnail images" in the bookshelf view.
All my books are epub.
All of them are created in Calibre.
Therefore all use jpg as the cover image format.
All of them display the cover correctly when I open the book in AdobeViewer.
However, some of them (a minority) do not display the cover in either the Thumbnails view or the Details view.
I did some testing a couple of months ago with one of these 'no-thumbnail' books (source was HTML file). I used the same cover image throughout the testing.
I reduced the book content down to a few chapters and created an epub in Calibre. When I sent it to the PB360, lo & behold, the thumbnail appeared. So far, so good.
I then put all the content back in the HTML and reconverted to epub. When I re-sent this to the PB360 (overwriting the reduced epub with the same name) I still had a thumbnail
Even better.
Then I deleted the epub on the PB360 and re-sent again from Calibre. This time back to no thumbnail
I'm ashamed to say, I gave up at this point and switched to a different bookshelf view.
P.S. There is no DRM on any of these epubs.