Downloading cover art without changing other tags

[ohrenkino]

See the documentation on handling the web sources:

Import Tags from Online Tag Sources

Mp3tag allows for importing tags and cover art from two online databases — Discogs and MusicBrainz — via Tag Sources → Discogs and Tag Sources → MusicBrainz. The last used Tag Source can be triggered by ⌘ I. The import works on a per-album basis and is usually a 3-step process: Search via Tag Source Here, you provide artist and/or album details to search for the release in the online database. The more complete, the better the result. [Tag Sources - Search] Choose one of the search resul…

Have a special look at the part which tells how to disable tag field information.
In your case only the cover should be ticked.