Regarding the soundcloud API.. when posting PLAYLISTS.. is it possible to provide artwork? artwork_data does not seeem to be a valid property when uploading?
Yup, just provide the playlist[artwork_data] parameter. If you were doing it with curl for example:
curl -D - -X POST https://api.soundcloud.com/playlists.json -F'oauth_token=foo' \
-F'playlist[title]=New Album' -F'playlist[sharing]=private' \
-F'playlist[artwork_data]=@test.jpeg' -F'playlist[tracks][][id]=trackid'
I'll update the docs to make this more clear, as I notice that artwork_data is missing from the list of parameters in the documentation of the playlist endpoint.