This article explains how to add album art to iTunes and Apple Music. Instructions apply to iTunes 11 and later on a Mac or Windows PC, except where noted.
Tutorial: How to Add Album Art in iTunes
There are a few ways to get album art, but the easiest way is to use the album art tool that’s built into iTunes and Music. The built-in album art tool scans your music library and Apple servers. When it finds artwork for songs you have, regardless of where you originally got the songs, it adds it to your library. Here’s how to use this tool to get album art in iTunes and Music.
The process depends on the program you are using:
In some versions of iTunes, a dialog will tell you that you need to send information about your library to Apple to get album art, but that Apple does not store that information. There is no way around this. Apple needs to know what music you have in order to send you the art. If you want to proceed, select Get Album Art.