I believe Tove Jansson also painted the color cover for the first authorized American edition of Fellowship of the Ring (1965). In _Letters_ one can read Tolkien’s reaction. He was quite taken aback, calling out the Emus (!) in the foreground. As if “ the illustrator hadn’t read my book.”
The 1973 Ballantine editions carried only Tolkien’s own paintings on the covers and slipcase.
I only learned of her involvement after becoming a huge Jansson fan. I had to take another look at the Emu.
It seems those were drawn by Barbara Remington [1] who didn't have time to read the books before making her illustrations. Apparently, the Lord of the Rings was in public domain in the US for a short while, so the publisher (and Tolkien) had to rush for a new 'authorized' version.
[1]: https://arrgle.com/emus-and-piracy-the-story-behind-the-lord...