Any standard YouTube URL works — youtube.com/watch?v=VIDEO_ID, youtu.be/VIDEO_ID, and youtube.com/shorts/VIDEO_ID are all supported, including share links.
Why is the max resolution thumbnail not showing?
Not every YouTube video has a maxresdefault (1280×720) thumbnail. This is common for older or lower-quality uploads. In that case, use the HQ (480×360) or MQ (320×180) option, which are available for all videos.
How do I actually save the thumbnail?
Click "Open in New Tab" for the resolution you want. Once the image opens, right-click it and choose "Save Image As" to download the JPG file. On mobile, long-press the image to get the save option.
Can I use the thumbnail commercially?
YouTube thumbnails are owned by the video creator and protected by copyright. You may use them for personal reference, but any commercial use or redistribution requires permission from the original creator.