Thumbnail Save is a free online media application which allows you to view and download any Youtube thumbnail preview image. Currently supported formats: YouTube (HD, HQ, 1080p, 4K), This free and fast thumbnail grabber allows you to save any YouTube thumbnail on your computer, cell phone, PSP, iPhone or nearly any other device for future use.
How to grab and download a YouTube thumbnail preview image:
1. Paste your YouTube URL in the box and press Submit.
2. Right-click the image and select "Save image as..."