Low cover resolution in ePub

Hi - I'm using Calibre to convert my .docx into ePub format for distribution. When I add a high resolution 2500 x 1600 (600dpi) JPG cover image to the file and convert to ePub the corresponding cover looks terrible in any reader I view in it, and the converted "cover.jpg" is shrunk and down to 96dpi. How can I improve the quality of the cover in the final ePub file?
Any advice will be great - thanks.