When you have a box set, it is common for each medium to come in its own case with its own cover art. If a box set contains a discography, the cases will probably match the artwork of the original releases while the box as a whole has its own art.

On services like Bandcamp, some albums will have a different piece of art. This could be on every track like this one (https://musicbrainz.org/release/6bc3847a-4e6f-47c4-897e-bf0d02b71514) or just on the bonus track like this one (https://musicbrainz.org/release/3ae239ee-6caa-40d9-97b5-f67c6cc492ee).

We currently mark this in the comments for an image, but different editors have different ways of marking this, and there is no way for this informal system to work across multiple languages. In both cases, we should be able to establish a proper link from the piece of artwork to the corresponding medium. This would allow better cover art handling for tools like Picard and LB.