Spotify Album Cover URLs are of the form https://i.scdn.co/image/ab67616d0000 + {quality identifier} + {image id}
The default seems to be b273, providing a ~640px JPEG. To get a different size or format, use one of the identifiers (maybe not exhaustive).
WEBP:
01e7: 64x64
11eb: 64x64
54be: 64x64