For example:
http://soundcloud.com/mhiqu3/sets/heretik/
How to get the thumbnail url: http://i1.sndcdn.com/artworks-000004897289-5r6ws9-large.jpg?1767646
You can use the /tracks endpoint, and read the artwork_url property from the returned JSON. More details: http://developers.soundcloud.com/docs/api/tracks
/tracks
artwork_url