I use width="100%" in all my images, and I have never seen a bad result in an Kindle, Fire, or emulation.
The narrowest Kindle AFAIK is the Fire tablets with a height/width ratio of 1.6. So any image skinnier than that (say 800x1280 pixels) ought to work just fine at height="100%"
Why wouldn't the Sigil default for a cover image (adapted for your use) work?
<div style="text-align: center; padding: 0pt; margin: 0pt;">
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 1800 2700" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink"><image height="2700" width="1800" xlink:href="../Images/YOURIMAGEHERE.jpg" /></svg>
</div>