[fred_be9300]

Originally Posted by PoP

Ahah, a new insight in cover view! Would anyone know the specs for that special thumbnail and special directory?

I wrote that "new insight", although it was hardly new
What I know about the PW covers, I mostly learnt from the forum here, with the origin probably this thread on the Calibre forum.

https://www.mobileread.com/forums/sho...d.php?t=193805
https://www.mobileread.com/forums/sho...d.php?t=192561

The thumbnails folder is system/thumbnails
The thumbnail filename rule I dont' know for sure. Seems to use the UUID or ASIN or something. You could check the Calibre source code (or ask over there)
The thumbnails must be max 330 px high (or so - I don't know the exact limit). Probeably there's a width limit too.

If I remember correctly, (I read that stuff a month ago), the Kindle Touch uses a similar approach, but needs even smaller thumbnails.
OTOH, the Android apps for instance, happily retrieve the cover from within the .mobi file.

HTH