bopgun

If you go to the plugin folder nextgen-gallery/view/gallery.php

Step 1: Find the code:
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<!-- Thumbnails -->
	<?php foreach ( $images as $image ) : ?>

	<div id="ngg-image-<?php echo $image->pid ?>" class="ngg-gallery-thumbnail-box" <?php echo $image->style ?> >
		<div class="ngg-gallery-thumbnail" >
			<a>imageURL ?>" title="<?php echo $image->description ?>" <?php echo $image->thumbcode ?> >
				<?php if ( !$image->hidden ) { ?>
				<img title="<?php echo $image->alttext ?>" alt="<?php echo $image->alttext ?>" src="<?php echo $image->thumbnailURL ?>" <?php echo $image->size ?> />
				<?php } ?>
			</a>
		</div>
	</div>

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Step 2: Replace above code this the following code

<!-- Thumbnails -->
	<?php foreach ( $images as $image ) : ?>

	<div id="ngg-image-<?php echo $image->pid ?>" class="ngg-gallery-thumbnail-box" <?php echo $image->style ?> >
		<div class="ngg-gallery-thumbnail" >
			<a>imageURL ?>" title="<?php echo $image->description ?>" <?php echo $image->thumbcode ?> >
				<?php if ( !$image->hidden ) { ?>
				<img title="<?php echo $image->description ?>" alt="<?php echo $image->alttext ?>" src="<?php echo $image->thumbnailURL ?>" <?php echo $image->size ?> />
				<?php } ?>
			</a>
			<span><?php echo $image->alttext ?></span>
            <span><?php echo $image->description ?></span>
		</div>
	</div>

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Note:
This is the code to add the description
<?php echo $image->description ?>

This is the code to add the title
<?php echo $image->alttext ?>