Mello

投稿にサムネイルが設定されているかチェックします。 
サムネイルが設定されていない場合、自動でフリーの画像素材サイトからランダムで画像をダウンロードしてサムネイルに設定します。 
ダウンロードする素材サイトはpixabayです 
ダウンロードできる画像の種類は100種類前後 
負荷対策で投稿数が多い場合は時間がかかることがあります
Make sure thumbnails are set for posts. If there are no thumbnails in the post, download the image randomly from the free image material site and set it as a thumbnail. 
The image to be downloaded is acquired from pixabay. 
Pixabay is a free material site that can also be used for commercial purposes.
The image to download is random 
The downloaded image can be checked from the media. 
If there are many posts, the thumbnail images may be duplicated. 
There are about 100 images to download. 
We will execute it at regular intervals with load balancing to the server. 
Recommended environment is PHP 7.0 or higher.
<h3>Arbitrary section 1</h3>

Thumbnail_Create

記事のサムネイルを自動で作成。画像を素材サイトから自動でダウンロードします。

Samuel Aguilera

NOTE: This plugin functionality is guaranteed by more than 10,000 active installs. But before installing this plugin bear in mind that its only purpose is to ADD the feature image ID to your post (in the same way that you would do using WP editor), it does not remove ANYTHING after deactivation. 
Before asking for support please read <a href="https://wordpress.org/plugins/easy-add-thumbnail/faq/" rel="ugc">FAQ</a> and <a href="https://wordpress.org/support/topic/please-read-before-posting-4" rel="ugc">this support thread</a>
<h4>How it works?</h4>
Checks if the post (any post type with thumbnail support, including pages) has already a featured image associated, and if not sets it using one of the following methods:
<ol>
<li>
Dynamically, for old published posts, the featured images are set only when needed to show them in the frontend. This means that the featured image is set (only first time) when a visitor loads the page where it needs to be shown.
</li>
<li>
For new content, the featured image is set in the publishing process.
</li>
</ol>
No options page to setup, simply install and activate.
If you want to exclude certain post type (e.g. pages), you can do it by using a filter. See <a href="https://wordpress.org/plugins/easy-add-thumbnail/faq/" rel="ugc">FAQ</a> for more details.
Easy Add Thumbnail doesn’t add any plugin data to your WordPress database. It simply adds the _thumbnail_id meta key with the ID of the attachment to be used as feature image (just the same that WordPress does when you set it manually).
Therefore it requires to have attached/uploaded at least one image to the post. If there’s not any image attached to the post, this plugin can’t help you.
Easy Add Thumbnail has not any control over how/when/where your featured image is displayed in your site, the display of the featured image in your site is TOTALLY controlled by your theme, this includes size, position, display it or not…
<h4>Requirements</h4>
<ul>
<li>WordPress 4.7 or higher.</li>
<li>PHP 5.6 or higher.</li>
</ul>

Easy Add Thumbnail

Automatically sets the featured image to the first image uploaded into the post (any post type with thumbnail support). So easy like that...

Thumbnail_Create vs Easy Add Thumbnail