List a first gallery image

Hello, I need to know if it is possible for me to list the first image of each linked gallery in a custom post type I created. I created a sales site where users will add images and I want to get one of those image to leave as a featured image in the post.
I use the following code in my single.php to list all post photos:

<?php get_header(); ?>

<div class="list-fotos image-gallery thumbnails-4 linked">
	<h2><?php the_title(); ?></h2>
	<span><?php the_time('d/m/Y') ?></span>
	<?php $images = easy_image_gallery_get_image_ids(); ?>

	<ul>
		<?php if($images) : foreach ( $images as $attachment_id ):?>
			<?php $imagefull = wp_get_attachment_image_src ( $attachment_id, '' ); ?>
			<?php $image = wp_get_attachment_image_src ( $attachment_id, 'thumb-custom' ); ?>
			<li>
				
				<a class="popup" rel="fancybox[group]" href="<?php echo $imagefull[0]; ?>">
					
					<img src="<?php echo $image[0]; ?>">

				</a>

			</li>
		<?php endforeach; endif; ?>
	</ul>
</div>

<?php get_footer(); ?>

In my index.php I want to grab a photo from the gallery and leave it as a featured image, how can I do that?