I love those mad Ballantine covers, they're almost like Hieronymous Bosch paintings!
I've also got a boxed set from the 80's with cover art by Roger Garland.
This site shows some of his artwork. The 'Tolkien Landscape' is used as the cover art for the books I have, broken into three, and the 'Gollum' print is also on the outside of the box. The second picture on this webpage, 'Middle-earth' is the art on one of a set of four tapes I have of Tolkien reading aloud and other excerpts. I think these are his best three pictures.
And I've just found this
Artists of Middle-earth site which sems to have a lot of things on it so I'm off for a look now...