I imagine it's the Tag Sources script author that controls whether or not cover art is fetched when using the Tag Sources feature, but I think it could be useful for the user to be able to override this behavior.
The reason being is this: when I search for an album, I'm first presented with a list of matches (which is usually quick being only text), but when choosing an album by pressing Next, the speed at which total album information is downloaded can vary from one to the other, and I believe this is because of the varying cover art filesizes.
Would it be possible for the user to override this, thus speeding up tagging on slower connections? Maybe there are other related reasons for the slowdown, but I'm thinking it's my connection speed.
Ideally, one could re-enable fetching cover art (via the cover art context menu / a checkbox) right on the fetched album information page ("Adjust tag information"), and then Mp3tag would request a re-fetch, this time with cover art.