For example:
http://soundcloud.com/mhiqu3/sets/heretik/
How to get the thumbnail url: http://i1.sndcdn.com/artworks-000004897289-5r6ws9-large.jpg?1767646
what you want to do is use Soundclouds API like so:
SC.get(PATH, function (track, err){
IMG_URL = track.artwork;
});
you can see an example here:
http://runnable.com/UuiMCfATvMMkAAAS/get-a-tracks-artwork-on-soundcloud-in-javascript#
the PATH to the track can be obtained in different ways
1. if you know the image id you can grab it like so
var PATH = "/tracks" + id;
2. if you know the name and user of the track
var PATH = "/users/dj-faze/tracks/phase-shift-360-019-degrees"
3. you can also get the id with other soundcloud API's
NOTE: Ensure the track's image you are trying to get is public. If it is private your going to have to log in with soundcloud authorization API.