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。

Kyle Gilman

<h4>A plugin to make video players, thumbnails, multiple resolutions, and video galleries.</h4>
This video plugin adds several options to any video uploaded to the WordPress Media Library. If your video can be played natively in your browser, or if you have FFMPEG installed on your server (optional), you can generate thumbnails from your video. Using either the “Generate” or “Randomize” buttons will create a selection to choose from. Click “Insert into Post” and you’ll get a shortcode in the post editor that will make a flexible, responsive video player.
If you provide multiple H.264 resolutions, Videopack can automatically select the one closest to the size of the player or a resolution of your choice, and provide a button for users to select the resolution manually. If FFMPEG is installed on your server, Videopack can encode the videos automatically.
You can also use Videopack to create a popup video gallery. The shortcode uses options similar to the <a href="https://codex.wordpress.org/Gallery_Shortcode" rel="nofollow ugc">WordPress image gallery shortcode</a>. In its simplest form it will create a gallery of all videos attached to the post.
You can now add advertisements to your videos using the <a href="https://www.videopack.video/add-ons/videopack-ads/" rel="nofollow ugc">Videopack Ads</a> premium add-on which you can purchase from the Add-ons tab of the Videopack Settings page or on the <a href="https://www.videopack.video/add-ons/videopack-ads/" rel="nofollow ugc">Videopack website</a>.
Not compatible with the Block Editor. Please continue to use the <a href="https://wordpress.org/plugins/classic-editor/" rel="ugc">Classic Editor</a>.
Visit the <a href="https://www.videopack.video/docs/" rel="nofollow ugc">Videopack Documentation pages</a> for more info.

Videopack

Great solution for generating thumbnails from images. 
Thank you for the work put into this plugin.

Excellent

Makes video thumbnails, allows resolution switching, and embeds responsive self-hosted videos and galleries.

Thumbnail_Create vs Videopack