To get the release cover images by musicbrainz release id, just point to http://coverartarchive.org/release/{mbid}.
For example go to http://coverartarchive.org/release/c63b0c2e-aa56-4808-bf4f-14c9f6a9982d to get a JSON doc with metadata like paths to images for Crystal Castles’ III release.
Or you could populate a src attribute of an img tag with:

• full size http://coverartarchive.org/release/c63b0c2e-aa56-4808-bf4f-14c9f6a9982d/front
• large thumbnail http://coverartarchive.org/release/c63b0c2e-aa56-4808-bf4f-14c9f6a9982d/front-500
• small thumbnail http://coverartarchive.org/release/c63b0c2e-aa56-4808-bf4f-14c9f6a9982d/front-250

Via MusicBrainz