Every video uploaded to YouTube is stored with a fixed set of thumbnail images. You do not need an API key to reach them: they sit at predictable public addresses built from the eleven-character video ID. The thumbnail downloader on this site simply assembles those addresses for you.
maxresdefault.jpg — 1280×720
The full-resolution thumbnail and the one you almost always want. It is only generated when the source video was uploaded in high definition, so older uploads and low-resolution videos may not have it at all. When it is missing, the grabber hides that tile rather than showing a broken image.
hq720.jpg — 1280×720
A second HD file at the same dimensions. It is less well known, and usefully it often exists on videos where maxresdefault does not — so it is worth checking before settling for a 640-pixel version.
sddefault.jpg — 640×480
Standard definition, present on almost every video regardless of upload quality. Note the 4:3 shape: the 16:9 image is letterboxed inside it, so the file carries black bars along the top and bottom that you will need to crop off.
hqdefault.jpg — 480×360
The size YouTube itself uses in most embedded players and search result grids. Also 4:3, also letterboxed.
mqdefault.jpg — 320×180
A genuine 16:9 file with no bars, useful for lists, link previews and anywhere the HD version would be wasteful. Too small to enlarge.
default.jpg — 120×90
The smallest size, used for compact sidebar thumbnails. Rarely useful on its own.
Which one to pick
For design work, a mood board or anything you intend to crop, start from maxresdefault or hq720. Scaling a large image down preserves detail; scaling a small one up does not, and enlarging the 120×90 file to poster size will look exactly as rough as you would expect.
Once you have the file, the resize and compress tool will take it to an exact size — including YouTube's required 1280×720 under the 2 MB upload limit — without uploading it anywhere.
A note on 4K thumbnails
1280×720 is the ceiling. YouTube does not store a larger thumbnail, so any site advertising a 4K YouTube thumbnail download is upscaling the HD file with software. That produces a bigger file, not a more detailed one.