I use an SVG wrapper for full page pictures. However, I've heard that some devices (kindle??) might have problems with that if it is other than the cover image.
Code:
<body style="margin:0; padding:0; text-align:center; background-color:black">
<div style="margin:0; padding:0">
<svg
xmlns="http://www.w3.org/2000/svg"
height="100%" width="100%"
preserveAspectRatio="xMidYMid meet" version="1.1"
viewBox="0 0 600 1050"
xmlns:xlink="http://www.w3.org/1999/xlink">
<image width="600" height="1050" xlink:href="../Images/img_008.gif"/>
</svg>
</div>
</body>