OK I never opened a PostGreSQL database before but was not afraid! 
I installed several clients they did not work until I saw your cool advice on how to open musicbrainz database for query..
Then I did \dt *.* to list all tables in all schemas.
I saw cover_art_archive.cover_art but it’s empty indeed.
cover_art_archive.art_type and cover_art_archive.image_type for instance are not empty.
On a side note do you know why select * from cover_art_archive.image_type doesn’t wotk with psql?
It seems that you are all using psql.
I kept using pgcli because it works there with same commands… 
psql queries only work on tables in default musicbrainz schema.
@InvisibleMan78 or @dns_server, or someone with a full DB:
Could you give me the content of (I don’t know the exact syntax, my table is empty):
select * from cover_art_archive.cover_art where release = 2117950
I will then try to insert the same 2 rows in my empty table (2117950 is the release we saw above).