Thumbnail issue post compression

*THESE ARE PLACEHOLDER IMAGES

Whenever I convert any video files (mp4, wmv, avi) into MKV the thumbnail is gone. Is there any setting that I might prevent this from happening?

If not is there any other software that helps you get these thumbnails (post conversion)?

*THESE ARE PLACEHOLDER IMAGES

Whenever I convert any video files (mp4, wmv, avi) into MKV the thumbnail is gone. Is there any setting that I might prevent this from happening?

If not is there any other software that helps you get these thumbnails (post conversion)?