Dave Ligthart

A simple way to choose a default thumbnail for your post.
Works alongside the featured image core functionality.
For theme usage:
<h4>Add this function where you want the thumbnail to appear</h4>
<pre><code><?php if(function_exists('wct_thumb')){ wct_thumb(); } ?>
</code></pre>

WP-Choose-Thumb

A simple way to add a default thumbnail to your post.

Sajid K.Shaikh

The Widget with some options. This plugin is based on the well-known WordPress default widget ‘Recent Posts’ and enhanced to display thumbnails of the posts.
The thumbnails will be built from the featured image of a post content. If there is neither a featured image nor a content image then you can hide the thumbnail.
The thumbnails appears left-aligned to the post titles. You can set the width and heigth of the thumbnails in the list.
The widget and shortcode is available. You can insert through widget on any sidebar any where easily.

WP-Choose-Thumb vs Recent Post Lazy Load