I submitted this idea on the Ubuntu Brainstorm site, and thought I'd link to it here as it may affect you. I don't know whether this could be addressed in Calibre or in Ubuntu itself (ebook files aren't "readable" in default installation) but please vote for the idea if you like it.
At the moment ebook files, other than PDFs, have a boring thumbnail based on their filetype. Whereas PDFs, PNGs etc. display the front cover, so at the moment the behaviour is not consistent
p.s. Sorry if this is on the incorrect thread, please move if it is.