You've only set a small handful of ebook-convert options. There are many options that can be set, including several around setting a cover. Try doing a conversion from the GUI and then go to the conversion log, which can be accessed by clicking on 'Jobs' in the the lower right of Calibre's main window. The top of the log will list out all the options that were enabled during the conversion - each option maps to an ebook-convert argument.
At a guess you're missing the --cover option and you're ePub doesn't include a cover.
Read more here:
http://manual.calibre-ebook.com/cli/ebook-convert.html