I “abused” the description property of the gallery.
Code adjustment in view/gallery.php:
<div class="ngg-gallery-thumbnail" >
<?php if ($gallery->description != 'nolink') { ?>
<a href="<?php echo $image->imageURL ?>" title="<?php echo $image->description ?>" <?php echo $image->thumbcode ?> >
<?php } ?>
<?php if ( !$image->hidden ) { ?>
<img title="<?php echo $image->alttext ?>" alt="<?php echo $image->alttext ?>" src="<?php echo $image->thumbnailURL ?>" <?php echo $image->size ?> />
<?php } ?>
<?php if ($gallery->description != 'nolink') { ?>
</a>
<?php } ?>
</div>