Xara adds a <DIV> the is the width of the viewport and places a background image (your picture) and set up some CSS, background-size: cover or centre 0px repeat, and sets its depth as z-index=1.

If you add:
<style>
.xr_strb {
z-index: 99;
}
</style>
to the page HTML Code (head), this should bring your stretch or tile to the front of everything.

Acorn