The Image Manager can be found in a media under Details > Image Manager. Here you can edit the Thumbnail of your video.
You can see 2 automatically generated thumbnails, but you can also upload image files for the thumbnail or capture a thumbnail from the video.
If you choose to upload a new thumbnail click on Upload files and find the Thumbnail image that you want to use in the video. Once you have uploaded the file you can choose it in the Image Manager.
Another alternative is to capture a thumbnail from the media. Go to video Details > Image Manager > click on "Capture from video". Now you can scroll the video to a time with a suitable frame and press "Capture this frame" and "Add to thumbnails".