How do I change incorrect album art on the default Music Player of the Samsung Galaxy S III?
What worked for me, as adx suggested, was to go into the /Android/data/com.android.providers.media/albumthumbs folder on the phone and delete all of the files it contains. They are extension-less files, so it is impossible to tell which one is the offending one, so I just deleted them all. The next time I opened the default Samsung music player, all of the album thumbnail images at first appeared empty, then after two or three seconds, the music player re-generated all of them, and my new album art appeared on the offending album.

(This was after I "showed hidden files and folders" via the control panel in Windows and deleted the AlbumArt.jpg and Folder.jpg files that were there)