function openCenteredWindow(url, aWidth, aHeight) {
     var aLeft = (window.screen.width-aWidth)/2;
     var aTop = (window.screen.height-aHeight)/2;
     return window.open(url, "_blank", "top="+aTop+", left="+aLeft+",height="+aHeight+",width="+aWidth+",status=no,toolbar=no,menubar=no,location=no,scrollbars=no");
}

function showImage(url, width, height) {
    openCenteredWindow(url, width+20, height+20);
}