Video Thumbnails simplifies the process of automatically displaying video thumbnails in your WordPress template.

Noted That:

To install correctly this video-thumbnails-reloaded.zip.
Fisrt Download the video-thumbnails-reloaded.zip to your computer
Extract/Open video-thumbnails-reloaded.zip to Your Computer.
Then, Find readme.txt file inside video-thumbnails-reloaded.zip and Open readme.txt.
Now, Read the Requirements of this plugin. Which Wordpress Version and PHP Version are required to run this Plugin in Your Wordpress Site.
Then, Follow the the Tips Below.

Start the Tips:

1. First Download "video-thumbnails-reloaded.zip" Plugin to your Local Computer. (Click Download)

2. Then, Login to your "yourdomain.com/wp-admin" Dashboard.

3. Then, Click on "Plugins" + "Add New" from left sidemenu of Dashboard.

4. Now, Click on "Upload Plugin" button.

5. Now, Browse "video-thumbnails-reloaded.zip" Downloaded plugin from your computer, Where you downloaded video-thumbnails-reloaded.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Video Thumbnails Reloaded" folder is added on left sidemenu. Now, Click on "Video Thumbnails Reloaded" folder.

Noted that: If you do not see "Video Thumbnails Reloaded" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about Video Thumbnails Reloaded Configurtions and Settings or How to work "Video Thumbnails Reloaded" in your WordPress site.

After Activated Plugin According to Step-6 then,

Go to "Plugins" + "Installed Plugin" from Wordpress Admin Panel Leftside Menu.
or Direct go to: https://yourdomain/wp-admin/plugins.php
Then, Find "Video Thumbnails Reloaded" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is Video Thumbnails Reloaded
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide

Guide
Upload the `/video-thumbnails/` directory to the `/wp-content/plugins/` directory Activate the plugin through the ‘Plugins’ menu in WordPress Some functions are available to advanced users who want to customize their theme: will echo a thumbnail URL or the default image located at `wp-content/plugins/video-thumbnails/default.jpg` if a thumbnail cannot be found. Here is an example: will return the thumbnail URL or return NULL if none is found. In this example, a thumbnail is only shown if one is found: `"; } ?>`

Upload the /video-thumbnails/ directory to the /wp-content/plugins/ directory
Activate the plugin through the ‘Plugins’ menu in WordPress

Some functions are available to advanced users who want to customize their theme:

will echo a thumbnail URL or the default image located at wp-content/plugins/video-thumbnails/default.jpg if a thumbnail cannot be found. Here is an example:
will return the thumbnail URL or return NULL if none is found. In this example, a thumbnail is only shown if one is found: "; } ?>

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Reffered: https://wordpress.org/