I'd try "max-width:100%; max-height:100%;". That should work (if the relative height works), but won't enlarge images, just reduce them if needed.
If you want to resize in any direction, I believe you have to use an SVG wrapper:
Code:
<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"
width="100%" height="100%" viewBox="0 0 600 800" preserveAspectRatio="xMidYMid meet">
<image width="600" height="800" xlink:href="images/Cover.png" />
</svg>
In viewBox and the <image> element use the image's pixel dimensions (not compulsory, but easier to manage). The key part is the preserveAspectRatio="xMidYMid meet" property.