YouTube thumbnail downloader
Paste a YouTube link or video ID to see every thumbnail size the video has, then download the one you need. Handy for studying what works before you design your own at YouTube's documented 3840 by 2160.
Which thumbnail size should I grab?
Take maxresdefault when it exists. It is 1280 by 720, the largest image YouTube serves at a predictable thumbnail URL. Below it, sddefault is 640 by 480, hqdefault is 480 by 360 and mqdefault is 320 by 180.
One naming trap is worth knowing: the hqin hqdefault means high quality, not high definition. Plenty of tools label that 480 by 360 image the HD version, which it plainly is not. If you want the sharpest copy of someone else's thumbnail, maxresdefault is the only one that qualifies.
Making your own thumbnail
Designing your own is a different question, because YouTube documents a larger target than it serves back through these URLs: 3840 by 2160 at 16:9, with a minimum width of 640 pixels, in JPG, GIF or PNG. The file limit depends on where you upload from, 2 MB in the mobile app and 50 MB on desktop. Our YouTube thumbnail size page has the full rules and a resizer preloaded to that size.