: Windows File Explorer does not natively display embedded MKV thumbnails. You can use Icaros to enable thumbnail previews for MKV cover art.
To add or view a "piece covering" (cover art or thumbnail) for the video file , you can use specialized media tools to embed an image directly into the Matroska (MKV) container. How to Add Cover Art to an MKV File
Click to save the new version of the file.