Within MusicBrainz different versions of the same release can all be grouped together under a Release Group. For example a CD version and an LP version of the same release would be part of the same release group. Artwork is added at the release level but usually the artwork for all releases within a release group is the same or very similar. Because of this if no artwork can be found for the release that SongKong matched to then SongKong will check releases that are part of the release group and if any artwork found use that instead.
Discogs has a similar concept to Release Groups called Masters. Masters also nominate one release as the Main Release which is often the chronologically earliest. But currently most releases in Dicogs are not linked to Masters so it isn't terribly useful to SongKong at the moment.
Instead we search for simply search for releases with the same artist and release name as our matched release and check the artwork of these releases. At least that is what it was meant do to but it wasn't working, now fixed in SongKong 2.1.1 so the artwork count should continue to rise.