How can I display embedded artwork on imported RIPd music albums in Apple Music on iPhone 12?

I have a lot of RIPd CDs. These are ripped to M4a apple format. The artwork is embedded in them at 500 x 500 pixel. Using Apple Music on my PC I have added the folder (album) then using Apple Devices Sync to my iPhone12. On the Windows 11 Pc in Apple Music the cover artwork is shown on Apple Music on the iPhone 12 the artwork is missing - just shows blank artwork cover image! Maybe Apple Music doesn't like M4a?? Or maybe the Artwork is too big at 500 x 500 pixels. On my Android phone using MP3 I embed the artwork at 300 x 300 pixels. The import works ok NOW I have figured out the way round the Apple system - on Android I just drag and drop! I would value some advice on what I need to do to get the artwork to appear in Apple Music on the iPhone 12. The music plays ok. Thanks

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