When using the Music Application, not all of the albums show the album's cover art. This is a problem because the artwork makes each album easily distinguishable from another.
The problem specifically occurs when a music file does not have the metadata that contained the original artwork. I've tried to ameliorate this problem by using third-party metadata-editing software to edit the music files' properties (.mp3/.ogg/.flac) to no avail. The S4 doesn't recognize these edits.
Previous iterations of the Galaxy allowed you to simply place a picture file of the artwork in the same folder containing an album's music files, but that solution does not seem to work.