I foolishly created a book with a cover image filename containing a space. This is written into the OPF (by Sigil) as:
<item href="Images/Dune%20Messiah.jpg" id="cover-image" media-type="image/jpeg" />
JTPepub's -cover method is unable to open the image, as it tries to open that literal name.
The filename in the zip file really does have a space:
95667 05-03-12 18:53 OEBPS/Images/Dune Messiah.jpg
The bug is that href values are URIs (not paths) which need decoding in order to be 'openable'. (So -textFromManifestItem: has the same problem.)