In MusicBrainz website display
As cover art has legal licensing and copyright issues associated, MusicBrainz can display cover art only from one of the approved cover art sites, linked to by a cover art relationship type or amazon relationship type.
In media files
Media players that support cover art display do so in several ways, often supporting more than one mechanism
- stored within the media player's own internal library/database
- as a separate image file in the same folder/directory as the release, using a naming convention (e.g. folder.jpg)
- embedded within each Track's metadata info (e.g. ID3).