The only way I know how to do this is to edit the plugin files (which will require making the change again each time you upgrade it). Here's what I did:
In the nextgen plugin dir, edit the file view/gallery.php.
Find the line of code for the anchor (link) around the thumbnail:
<a href="<?php echo $image->imageURL ?>" title="<?php echo $image->description ?>" <?php echo $image->thumbcode ?> >
Replace the title attribute with:
<?php echo $image->alttext ?>
Result:
<a href="<?php echo $image->imageURL ?>" title="<?php echo $image->alttext ?>" <?php echo $image->thumbcode ?> >
Explanation: The effects like the Shutter box use the anchor title attribute to display text under the image. NextGen defaults this to the description field, but the edit above will replace this default with the alttext field.
Hope that helps!