See a previous thread on this topic
You can use wikipedia / wikidata / wikimedia commons as a source of images.
It is on my plans for my wikidata plugin to download images from there.
Finding the url is simple but I should probably learn qt and create a preference page to enable this feature before I implement it.
There are other options as discussed in the above thread such as searching flickr for musicbrainz artist id tags and using that but I am not sure if there is any software that currently does that.