Welcome to the forum MashaChama.
Don't take this as some brush-off, but there is an enormous amount of information already available on this forum.
If you do a search on displaying album art I would be surprised if you don't find information that solves this for you.
The search feature of the forum is known not to be fantastic, so you may better use google.
('musicbee albumart not showing" or something similar)
edit:
I ignored 'somebody else posted while you were typing'. So, also what The Incredible Hidden Boom Boom says ;-)