An other solution (used in the Sigil User Guide btw) that seems to work correctly is:
Code:
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 600 800" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink">
<image height="800" width="600" xlink:href="../Images/cover.png"></image>
with height="800" and width="600" corresponding to the real size of the native image.