ulfben

This plugin adds an “Use as Featured Image”-action to the Media Library. Very useful if you need to set the post thumbnail / featured image for a lot of posts.
<a href="https://wordpress.org/extend/plugins/bulk-select-featured-image/screenshots/" rel="ugc">A picture says more than a thousand words</a>.
//<a href="https://www.ulfbenjaminsson.com/" rel="nofollow ugc">Ulf Benjaminsson</a> 
Copyright (C) 2012 Ulf Benjaminsson (hello at my full name dot com).
This program is free software; you can redistribute it and/or modify 
it under the terms of the GNU General Public License as published by 
the Free Software Foundation; either version 2 of the License, or 
(at your option) any later version.
This program is distributed in the hope that it will be useful, 
but WITHOUT ANY WARRANTY; without even the implied warranty of 
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 
GNU General Public License for more details.
You should have received a copy of the GNU General Public License 
along with this program; if not, write to the Free Software 
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

Bulk-Select Featured Image

Allows you to select Featured Image / post thumbnail for your posts directly from the media library view.

pjgalbraith

Automatically display a default Post Thumbnail image, either the first image attached to a post, or set default images for specific categories, tags, or custom taxonomies. Supports custom fields to ease upgrading.
Includes the SLT File Select plugin created by gyrus (http://www.babyloniantimes.co.uk/index.php?page=donate)
<h3>Documentation</h3>
See documentation at http://www.pjgalbraith.com/2011/12/default-thumbnail-plus/

Default Thumbnail Plus

Add a default thumbnail image to post's with no post_thumbnail set.

Bulk-Select Featured Image vs Default Thumbnail Plus