If you open the galleries folder, then expand the gallery that you are using, and then open the index.php page.
you will see a line that reads something like this:
<img alt="" class="loader-image" src="<?php echo $pg_config['html_prefix']; ?>images/loader_<?php echo $pg_theme; ?>.gif" />
You could see the alt tag is empty, so instead change alt="" to:
alt="<?php echo $pg_config['description']; ?>"
and then if you are using the ribbon styled one, then you will need the same change on the same page a couple of lines down.