On February 1, 2014 Discogs changed their terms of use. They now require authentication for all album art downloads and also limit total downloads to 1,000 per day. I am currently working on adding authentication support however the limit of 1,000 images per day is an application limit, not a user limit, so I'm not sure how that will work with Tag's user base.
I am also investigating adding MusicBrainz cover art support.