I had a problem with this too on the covers of epub format books a long time ago when I first started using Coolreader and my Nook Simple Touch. I think I found the answer on the forum here somewhere. It was pretty simple but it will involve some editing of the file to make sure that the metadata section of the content.opf file in the epub contains this line:
Code:
<meta name="cover" content="cover.jpg" />
where cover.jpg is a file in the epub's image collection. There are probably many ways to go about that, but I like using Sigil to edit stuff like that since I also clean out the CSS files in epubs and use my own. With that code and the picture file in the epub, the proper cover should show up inside Coolreader's library display. I'm only familiar with epub though, since that's the only ebook format I deal with.