MB offers custom web links feature in Preferences> Internet. By adding the custom web links to the track information panel, you can easily search album covers and artist thumb images for either selected or playing album.
This is for searching album covers:
https://www.google.com/search?tbm=isch&tbs=imgo:1,isz:l,iar:s&q=<Artist> <Album> Album
If you click on the web link, you will get something like this:
tbm=isch means image search, tbs=imgo:1 displays size caption, isz:l sets size to large, iar:s sets aspect ratio to square.
So if you do not need any of those parameter, simply remove it (along with a preceding comma). For classical music covers search, I strongly recommend you remove size and aspect ratio parameter from the query to get as many results as possible.
This will open a single tab in your browser even if you launch with multiple selected tracks. But if you're gonna mostly use with multiple selected tracks, use <Album Artist> in place of <Artist> in the template because it won't include artist value in the URL if selected tracks have different artist tag values. In that case it will only search with album value.
And the following template is for searching artist thumb images:
https://www.google.com/search?tbm=isch&tbs=imgo:1,iar:s,itp:face&q=<Artist>
A new parameter here is image type "itp" and it's currently set to "face". For some artists "photo" could generate more results and for others "Any" may work better. For a band with a too general name, add "Band" to the search term to get more relevant images.