This Links will guide you to your Youtube Thumbnail:
https://img.youtube.com/vi/<insert-youtube-video-id-here>/default.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/hqdefault.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/mqdefault.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/sddefault.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg
They are available, until your video will be removed.