As I'm new to MP3TAG, this feature may already be implemented, and I am merely revealing my ignorance of the interface, however it is not immediately obvious if it is possible to retrieve covers by URL, a useful feature in Clementine (though clementine's file update operations are more opaque and therefore less useful).
What I'd like to do is be able to google image search for album art (for esoteric items, far more useful than amazon or other dedicated sources as such art is often only curated on fan-based sites). Once the URL for the item is found, rather than downloading it to a local directory and then picking it up using the right click -> add cover (from local directory) option, it would be convenient to have a right click -> add cover from URL option.