alt tag for .backstretch in joomla template version 3.4

[erankal]

hello i have a website that build with joomla.
and its use the backstrech plugin as a defult for the template
i want to add the tag alt for the images but i dont know how to do that ?
i add some scripts from the index.php file where is the backstrech loaded.

     $('#bg').backstretch([
  "<?php echo $this->baseurl; ?>/images/zelcer/bg/img1.jpg "
, "<?php echo $this->baseurl; ?>/images/zelcer/bg/img2.jpg "
, "<?php echo $this->baseurl; ?>/images/zelcer/bg/img3.jpg "
, "<?php echo $this->baseurl; ?>/images/zelcer/bg/img4.jpg "

], {duration: 6000, fade: 750});
});

thanks Eran,