А почему проще не сделать:
<img src="http://mirror1/picture_10kb_567856.jpg" onload="imgloaded('http://www.xz1.com')" border=0>
<img src="http://mirror2/picture_10kb_345345.jpg" onload="imgloaded('http://www.xz2.com')" border=0>
var mirrored = 0;
function imgloaded(loc) {
if (!mirrored) { document.location.href = loc; mirrored = 1; }
}