Copyright and Reuse Considerations
The thumbnails are copyrighted by the video owners. To reuse them, you need permission, especially for YouTube videos. For other uses like websites and logos, reusing thumbnails likely won't lead to DMCA complaints.
Optimizing Reused Thumbnails for SEO
Simply reusing YouTube thumbnails is not optimal for SEO since Google indexes the originals. To make reused thumbnails SEO-friendly, modify them with Photoshop or other software. This makes them unique again for search engines.