Album Cover Art

Hey ! I just got my Y1 music player today, and I've put my existing music files on it (my last mp3 player died after just 4 months haha.)

I get my music files from a variety of places; CD ripping, spotify to mp3, and youtube to mp3 are the most common. Some of these make album art included, some don't. I was wondering if there was a way to put album art on preexisting mp3/flac files without redownloading/re-ripping them (such as a file to drop in the album folder, etc)

Please let me know if this has been or can be achieved !! I would prefer not to redownload half my albums haha.