In previous versions of iTunes, you could right-click a track and choose Get Album Artwork to have iTunes download artwork (if it was available) from the iTunes Store. With iTunes 12.4, this has changed. The Get Album Artwork no longer shows up in the contextual menu when you select a track.
To get album artwork now, you have to click the artwork area for an album, and you can do this in any view. Here’s how it looks in Albums view:
And here’s the menu item in Genres view (I’ve selected an album that already has artwork:
Note that this menu item only displays when you click on an album; it doesn’t display if you click on tracks, even if you select all the tracks of an album.
You shouldn’t use the File > Library > Get Album Artwork command, because that will check for artwork for your entire library, potentially replacing some of your artwork.