I have to say thanks after reading this thread. For the past few weeks I've been reading The Dresden Files, but I'm getting picky about ebook formatting so they all look the same, and decided to force everything to look like I want with Sigil. An annoying problem I kept running into though was that cover images would not show up in Cool Reader on my NST, and affected files would also break CR's current book screen saver feature too. It turns out it's because of a missing <meta content="cover.jpg" name="cover" /> in the OPF file. After adding that in, everything shows up, and I don't have to manually copy in a cover picture on the broken epubs for the screen saver.