I Saw Laughter

So, one of the ways I find new music is by downloading fanmixes (which is not quite as successful as finding a television show that has epic music, but I've still found some gems). Now, some skilled people are make covers to go with these fanmixes... how do I make it so that the images they made act as album art? (I have iTunes, btw.) Sometimes they do it automatically and sometimes they don't.