Check the image resolution in either by extracting the epub (it's just normal zip archive, you can extract it and browse images), or by using calibre ebook editor. If the image embedded inside ebook is low resolution, then I don't think there's much you can do about that.
@Ken Maltby There's new plugin in KOReader, that lets you set screensaver to last book's cover automatically in case you're interested
https://github.com/koreader/koreader...x-boox-devices