You can easily export a still image/video frame from your Media Composer clips to use as a thumbnail for your YouTube video, or any other purpose where you need to export a video frame to use as a graphic.
You can export several different formats of still images, JPG, PNG, and many more. If you want to use this graphic for your YouTube thumbnails, you can upload it as-is, or drop it into Photoshop or Affinity Photo for further editing and/or adding text.
It’s as easy as choosing what you want to export, park your play-head on the frame you want to export as a graphic, and then export. I am using Media Composer 2018.1 in this video.