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Note: It's basically the same on Windows than it is on Mac
Search for the album artwork and manually put them onto the songs or album

1. Get the artwork jpg png etc

2. Select the song and Right Click on Get Info



3. You're gonna get a window that looks like this:



Click on Artwork (circled area)



Drag and Drop the jpg onto the white box area or click on Add select the image and click OK

If you're gonna do multiple songs, Select the songs and Right-Click and click on Get Info

You're gonna be asked if you want to edit multiple items, click yes



Once you click yes you'll get a window like this:

and drag and drop the album artwork onto the circled area and click OK