The iPhone 12 does not let you add Album art, you must do it on your Mac in iTunes and then sync the audio files with the new art or photo to the iPhone.
Since the Support specialists were to busy to answer this I researched it and post it for others that don’t know how to do this:
How to Manually Add Album Art to iTunes.
iTunes on Mac can automatically find album artwork for you. Right-click on the album and select Get Album Artwork.
Here is how to add your own and personalize any playlist.
- Right-click on the album and select Album Info.
2. Another screen opens up with all the associated meta-data for the album or song. Go to
the Artwork tab next to the screen for Details.
3. Click on Add Artwork. This will open the Finder window in Mac. Select an image file or
drag the image file to the artwork area. You can select a downloaded file from the many
cover art sites on the web or make your own graphic.
Discussions on the web recommend a size of 600x600 px for the album art.
4. Click OK and you are done.
The album art is now uploaded to iTunes. Plug in your iPhone via USB cable and sync to your iPhone. Now you won’t have to look at the boring 2 music notes.