The first thing you need to do is to find out the release identifier (MBID release) for the album from which you want to receive images, you can do this using the search API, for example:
http://www.musicbrainz.org/ws/2/recording/?query=artist:nikes+recording:ftg2
Since this is a search, you will get many results, some of them will be erroneous. In this case, the first result is correct and corresponds to only one release, it has the following MBID: "c04730ea-87cb-478b-a256-08c0561d20e6".
Now that you have the identifier, you can do a search in coverartarchive:
http://coverartarchive.org/release/c04730ea-87cb-478b-a256-08c0561d20e6/
Which will give you a json answer with a list of available images (or 404 if this version does not have a cover in Coverartarchive).
Coverartarchive is fairly new, but may not always have a cover. MusicBrainz sometimes has a link to an art cover that you can search for, including βurl-relsβ in the webservice request, for example, for the MBID of the release βfdfdb3c0-8820-4c22-94e6-6aa136e36c31β you can run this request:
http://musicbrainz.org/ws/2/release/fdfdb3c0-8820-4c22-94e6-6aa136e36c31?inc=url-rels
And in the output, you will see the image URL:
<relation type="cover art link"> <target>http://liveweb.archive.org/http://www.8bitpeoples.com/images/albums/large/8BP049.gif</target> </relation>
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