Spotify cover art downloader

Sizes come from Spotify's own CDN naming, and each one is measured from the downloaded file rather than trusted from the URL. Albums, tracks, and podcasts are usually published at 640, 300, and 64 pixels; artist images at 640, 320, and 160. Sizes that do not exist for a given image are left out. There is no higher-resolution original behind any of this: Spotify publishes nothing larger than its biggest size, so the largest row here is the best that exists anywhere.