You can also wrap everything in SVG...but only the "cool" devices support SVG....
Code:
<div style="margin:0; padding:0; text-align:center">
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMin meet" version="1.1" viewBox="0 0 881 1500" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink">
<image height="1500" width="881" xlink:href="../Images/cover.jpg" />
<!-- add 100px to height of viewbox and image to make room for caption -->
<text text-anchor="middle" x="50%" y="40" style="fill:white; font-size:50px; font-family:serif">Caption</text>
</svg>
</div>