Flickr, the premiere photo sharing site, has a concept called Machine tags, which are machine readable tags applied to photos. These machine readable tags allow users on Flickr to tag photos so that they reference other resources on the net.
In order to tag photos of artists, photos of (covers or otherwise relating to) releases, tracks or labels, use the following convention:
For example, this Heartless Bastards photo has an MusicBrainz artist machine tag.
To search for musicbrainz tags, use these links: