add_action('admin_head-media-upload-popup','wpse_55240_required_alt_text');
function wpse_55240_required_alt_text()
{
echo <<<HTML
<script language="javascript" type="text/javascript">
jQuery(document).ready(function($) {
$("#TB_overlay").click(validateAltText);
$("#TB_closeAjaxWindow").click(validateAltText);
function validateAltText() {
var value = $(".image_alt input").val();
if (value)
return true;
alert('Please fill the Alt text');
return false;
}
});
</script>
HTML;
}