It would be nice if added files got deleted cover automatically and then inserted new cover.
It is not always that purchased music has a good cover.
Bliss almost always finds better cover.
But bliss just doesn't delete the cover automatic and insert a new cover.
You have to do this manually
Note that if the original cover is less preferable in a consistent way to bliss's you could define a rule that means bliss will replace the art.
For example, if the original cover is always 300x300 you could define a min size rule of 500x500 and ask that bliss always replace automatically.
But I realise this might not always be the case.