Hello precisao,
Thanks for using Apple Support Communities.
From your post I understand that you need to add artwork to some albums that you have imported into iTunes. Please follow the steps below to add artwork to those albums.
Do any of the following:
Add artwork to one item at a time:
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Select an item in your iTunes library and choose File > Get Info.
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Click Artwork and do one of the following:
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Click Add Artwork, select an image file, and click Open.
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Drag the art file to the artwork area.
Add the same artwork to multiple items: Select the items, choose File > Get Info, click Info, and drag an image file to the Artwork area.
Retrieve album artwork from the iTunes Store for songs in your library: Control-click the songs and choose Get Album Artwork.
iTunes 12 for Mac: Add artwork to songs and other items
Take care.