This is very interesting. I am indeed referring to the tendency started by Iron Maiden, particularly what impressed me was their cover for the album "Powerslave" (from which the only song I can remember is "Aces High...")
That started off my general interest in this subject. It looked so elaborate and interesting. Sadly, I regret to say I enjoyed the art far more than the actual album!
And with reference to Tolkein, a power metal band I know called Battledore are a good example of a band very heavily influenced by the imagery in his books. All their songs have direct references to his stories. Here's the album cover from their fourth or fifth album Evernight:
Look familiar? Falls of Rauros I think, I'm not an expert on the subject....