In the event you are not aware: 1st thing to understand is that MC allows you to apply cover art on a per track basis. Consequently, pasting cover art on one track does not automatically apply it to all the other album tracks.
If you want to ensure that whatever cover art image you are pasting will apply to every track for a given album, make sure you either: first select all that albums tracks, or paste the selected image on top of the album in an album view. Doing the latter will automatically apply the pasted image to all the album's tracks.
This approach should always work, however, if the track your pasting the image to is currently playing, it won't reflect the updated cover until you stop play, and refresh the view. Depending on what view you have active while performing the copy/paste, refreshing the view is often necessary to see the updated image.
One question I have is, do you need to update the cover art file in the source folder to update properly? When you just copy and paste from different files to replace the cover art in MC, it seems like the cover art file in the source folder does not get updated.
When I copy/paste, it always updates the source folder cover art image as well. That said, MC has several options for how you store the cover art. I don't think it would behave differently for the different methods, but it might. With that caveat...
For the record, I store all album covers in a single folder, but based on numerous posts I have read over the years, that seems to be the less used method. Honestly, I am not sure why that is though. I find having all the album art in a single folder enormously beneficial. In addition, MC
automatically uses the album tags to name the cover art image.jpg file as
Artist – Album Name. The only thing I would change is for MC to add the
Year before the album name - then it would be perfection.
Whatever the advantage is in having thousands of files identically named
folder.jpg in thousands of different folders eludes me. I would be interested as to why some users apparently feel it is superior to the single folder approach. I did try the
folder.jpg approach when I first started using MC 17+ years ago, but all I remember at this point is I found it painful to whatever I was trying to accomplish at the time. Maybe with the perspective of how others make use of it, and my much deeper knowledge of MC in general, I can be convinced it is the better method.