After being bugged by off-center images, I finally figured it out to add text-indent:0 to stylings for images. I wish it would be that way by default when you use "center", but oh well, live and learn.
IMHO the image should NOT be 100% of the screen size, but something slightly less
. I agree with this. When I first started fooling with epub, the only viewer I had was the Calibre Reader, and the cover image would always need two page-turns to advance to the next page. By trial and error, I learned to set the height to 99% and had no more trouble with the reader. I continue to do that as an habitual CYA.