As it currently stands, artist pictures are stored in these two folders:
The first one is the source for the images used in the view "Artists" while the other one is the source for the images used in the panel "Artist picture".
When pasting a custom image in the view "Artists" it only copies that file to the first location which means you end up with two different images of the same artist when using that view and panel together.
The way I've handled this so far is that I from time to time copy all the files from the first location, changes the extension to .dat and then replaces all the files in the second location. I could probably automate this but I'm curious as to the reason why this isn't handled natively. Is there any scenario I'm not aware of where it would make sense to have two different images?
My suggestion is simply that when an artist picture is pasted, the picture of that artist is also sent to InternalCache\ArtistBackdrops.