I can't really answer the question, although my guess is leaning towards Fėanor: there are three jewels, and the character is surely not evil-looking enough to be Morgoth at that point. That being said, I have the equivalent edition of
The Book of Lost Tales Part I and it has an equally ambiguous cover:
http://d.gr-assets.com/books/1403181958l/7347.jpg
One assumes it's Eriol speaking to one of the Elves, although I'm unsure which. I'm not very fond of it, I must admit. It makes it look like they're trying to market this fairly serious critical study of draft material as an 80s fantasy adventure novel (albeit one with lots of chatting). It reminds me of the covers to David Eddings' stuff, or the absurd covers of the American editions of
The Wheel of Time. "Eriol" also looks a bit bored.
My well-loved 1999 Harper Collins paperback edition of
The Silmarillion bears Professor Tolkien's painting of Taniquetil,
Halls of Manwė on the Mountains of the World above Faerie on the cover. I'm rather fond of that, by contrast. My editions of the three volumes of
The Lord of the Rings are in the same style and use Professor Tolkien's original cover designs, which I feel lends them a certain something.