For example:
http://soundcloud.com/mhiqu3/sets/heretik/
How to get the thumbnail url: http://i1.sndcdn.com/artworks-000004897289-5r6ws9-large.jpg?1767646
with the api (or without) ?
I know this is quite old, but I stumbled in this question looking for something else and I'd like to give my two cents.
These are all possible sizes (source API Reference):
* t500x500: 500×500 * crop: 400×400 * t300x300: 300×300 * large: 100×100 (default) * t67x67: 67×67 (only on artworks) * badge: 47×47 * small: 32×32 * tiny: 20×20 (on artworks) * tiny: 18×18 (on avatars) * mini: 16×16 * original: (originally uploaded image)