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
90d5: 64x64
ab87: 300x300
e1a3: 300x300
1a9d: 300x300
2d53: 300x300
a426: 640x640
aa54: 640x640
d619: 640x640
75a0: 640x640
JPEG:
745c: 64x64
4851: 64x64
f848: 300x300
1e02: 300x300
b273: 640x640
d452: 640x640
82c1: 1400x1400