I was looking through my music collection while hidden files were shown, and I noticed that almost every directory/album has a folder in it named
mediaartlocal, which contains a jpg file with an album cover thumbnail, having a name such as:
about 40K size.
It might be from Google's play music app, to perhaps locally cache the album covers, mainly because it's the only app I can think of that might want to do this kind of thing. But before I delete them all, I'd like to know where they come from.