There are many websites, which provides you with a good quality thumbnail. Using canva you can create your own thumbnail but as a beginner, if you want to download a thumbnail you can use this website, simply paste your youtube video link and save the thumbnails into your phone or laptop or desktop.
Youtube Thumbnail Downloaders is a Download YouTube Thumbnails website that is helpful to you Save or Download YouTube Video Thumbnails. YouTube thumbnail downloader allows you to download YouTube thumbnails easily and fastly. It's an online tool, not need to install any app, entirely free for anyone to use. You Can Download free youtube videos thumbnails images in Full HD (1080), HD (720), SD, and also in small size.
Download youtube thumbnail images of all quality for free. This application let you download thumbnails of all quality. Just paste the URL of the thumbnail video in the below input and click Get Thumbnail Image