When you rename folder.jpg, are you doing this in Explorer?
If so, should MM really remove a link to folder.jpg? It could just be that the file is inaccessible for some reason...
or are you saying that the file monitor should take care of this automatically?
Well, both filemonitor or add/rescan should remove it if is in-accessible (or renamed/moved in the case of file monitor).
Scenario A: user renames/moves/deletes album art image from explorer from monitored folder... file monitor should rename/move/delete image link as appropriate.
Scenario B: user renames/moves/deletes album art image from explorer from un-monitored folder or mm not running... add/rescan should rename/move/delete image as appropriate.
I would consider it proper behavior of the option > remove unused attributes (artist, album, composer, art)