How do you add a thumbnail to an MKV file?

[VideoFanatic]

Also after I add a thumbnail to the MKV if I then convert it to an MP4 will it retain that thumbnail? If not, is there a way to add a thumbnail to an MP4?

I tried MKVToolnix. I dragged the 1920 x 1080p MKV inside it. I dragged the 1080p thumbnail into the Attachments window.

For Name I entered "cover.jpg"
MIME Type: image/jpeg

I get an MKV but the thumbnail isn't added. Does the thumbnail have to be a specific size?

[davexnet]

See this page, it has info about embedding a thumbnail
https://video.stackexchange.com/questions/35598/is-it-possible-to-embed-a-thumbnail-in-a-mkv-file

[VideoFanatic]

That seems to be for Linux, I'm on Windows 7. I'm sure there was a way to do it on MKVToolnix as I managed to do it in the past but I can't figure it out.

[VideoFanatic]

I actually managed to change the thumbnail with the MP3Tag program as I can see the thumbnail change on the video in Windows explorer but when I upload the video to Twitter, it's still using the first frame of the video as the thumbnail.