I poked around my Tolkien collection, and I wasn't able to find an edition with a color plate of Tolkien's watercolor. I do have a color art card of the picture, but it is quite faded and so will not do.

If you do a Google image search with the words "Tolkien Lothlorien," you will find five or six different representations of the picture; I hope one of them will have a high enough resolution that you could use it? I'm afraid I don't have anything better in my own collection.