A free app for Android, by Demo Developer.
The best way to get high-quality thumbnails for videos is to download them from a website like YouTube, Facebook, Instagram, Twitter, Vimeo, or your favorite video hosting site. If you want to save time, you can download the thumbnails directly from the site itself and then use them in your videos.
You can download thumbnail files that you can open in any editor and start building your own channel. The thumbnails are available in HD, medium, or low quality. You can also generate tags for the video and save them for later use.