Musicbrainz doesn't store any images, you have to create relationships from the artist to an image or WIkipedia Commons.
(do this by hitting 'Edit' at the top of an artist page when you're logged in and hit 'Add Relationship')
outsidecontext's answer is pretty clear after that, do you have any specific questions about it/ how to link to an image?
Note that if you want MusicBrainz to display an artist image on the artist page, but the artist isn't known enough for you to make a Wikipedia entry about them/have something accepted to Wikipedia Commons, you're probably sh*t out of luck sorry to say. As far as I know.
You can add a relationship to any image, but MB wont display it on the artist page.