You should be able to fetch the artwork into a UIImage via the following:
MPMediaItemArtwork *itemArtwork = [mediaItem valueForProperty:MPMediaItemPropertyArtwork];
UIImage *artworkUIImage = [itemArtwork imageWithSize:CGSizeMake(64, 64)];
In essence, the MPMediaItemPropertyArtwork
property returns a MPMediaItemArtwork which you can then obtain a UIImage from.