wp_list_pages only displays page title and link in list. Try following code to fetch page content and thumbnail.
<ul class="pages-list">
<?php $our_pages = get_pages($args); ?>
<?php if (!empty($our_pages)): ?>
<?php foreach ($our_pages as $key => $page_item): ?>
<li>
<a href="https://wordpress.stackexchange.com/questions/147745/<?php echo esc_url(get_permalink($page_item->ID)); ?>"><?php echo $page_item->post_title ; ?></a>
<?php echo get_the_post_thumbnail($page_item->ID,'thumbnail'); ?>
</li>
<?php endforeach ?>
<?php endif ?>
</ul>