@yodykihara
As most readers have a screen of 600x800, your cover should be smaller and you have to edit the titlepage.xhtml file in epub and fix the dimensions there. With
<img src="cover.jpeg" alt="cover" style="height: 100%" />
your cover is stretched to the screen.
The height of your cover should not be in percent as above but fixed with (absolute height) like
<img src="cover.jpeg" alt="cover" style="height: 580" />
This is my cover with original size of 460x580 and the width is kept correctly, there are no distortions. I've got some empty space around the cover, but I don't mind, it looks better than the distortion. You may try it with another height between 580 and 600.
(See also my ticket 8668 enhancement).
George