There are really two things that I'd do to create this display flow; there are other methods, but I find this easiest to go back later and revise.
First, in the HTML image calls themselves, make sure that you haven't set width and height with anything except the "100%" parameter, which might look like (leaving the angle brackets off because they get screwy if you try to quote this):
Code:
img src="cover.jpg" width="100%"
which should leave the aspect ratio (proportions) the same. And avoid SVG if at all possible, too, because some of the results can be unpredictable.
Second, impose a style with the page-break-before parameter. This is best in a structure like this (again, leaving off angle brackets):
Code:
p class="imagemain"
img src="cover.jpg" width="100%"
/p
and the style imagemain looks something like this:
Code:
.imagemain {
page-break-before: always;
margin-top: 0;
text-align: center;
}
which, for a fully-HTML-and-CSS-compliant reader, would do the trick. (Unfortunately, none of the proprietary systems — Kindle, Kobo, iPad, etc. — are fully HTML-and-CSS compliant, so there Will Be Glitches.)