Ok, I had the same problem I've found where it is;
To update/change/add artwork to an album WITHOUT having to manually copy the art file to every song in an album you have to select all the songs in the album, like the old way, then cmd+I or get info and the normal 'limited' pop up box for multiple items appears. On the right (of the 'info' tab) is an album artwork box which you can drag images to in the same way as you could to the album art preview in the bottom left.
This had me very frustrated, but thankfully its still there, just an extra pair of clicks away. Not great for updating loads of albums quickly in a swift workflow but at least you can still add artwork to multiple items!