Try to do:
<script type="text/javascript">
$(document).ready(function () {
$("table").tablesorter({
sortList: [
[0, 0]
],
textExtraction: function (s) {
if ($(s).find('img').length == 0) return $(s).text();
return $(s).find('img').attr('alt');
}
});
});
</script>