This has to do with your YouTube video setting. Try setting a thumbnail in YouTube itself.
The featured image and the video don’t have a connection within Ghost. You’ll need to set the featured image separately from adding the video.
This site will make things a little bit easier for you by grabbing the thumbnail for you as an image: http://www.get-youtube-thumbnail.com/. But it doesn’t get around the issue of repeating the thumbnail. You might just need to create a separate featured image apart from the video (maybe two different screen captures?)
I had set a thumbnail in YouTube but I think maybe the video hadn’t finished uploading when I posted the link on Ghost, so that was why there was just a grey box. I’ve now updated the thumbnail and it works fine.
I think it’s probably better to just set a featured image that is completely different from the video, to avoid confusion (currently the featured image looks like a video, so people might mistakenly click on it and expect it to play). But then the featured image won’t let people know that this post is a video. Perhaps I will just add [VIDEO] before the post title so they know what to expect.
One other option for indicating that a post is a video is to use a tag. If you set the first tag to “video,” then that tag will be visible on the homepage.