I've seen a lot of work by Frank Frazetta featuring Burrough's fictional heroes, but I don't know if it was as cover art. Other than Frazetta, I don't know -- have never known -- the names of Burroughs' illustrators.
Comic book/commercial artist Neal Adams did covers during the 1970s for ERB's Tarzan series published by Ballantine Books. His action sequences are just awesome! The covers for the previous editions by BB were painted by Robert Abbett who was quite good.
Gino D'Achille painted the covers for the Barsoom books I own. Also a fine artist.
The Brothers Hildebrandt did some other BB editions of ERB's books.
Frazetta's ERB work is AWESOME!
But I have a true fondness for Joe Kubert's Tarzan of the 1970s published by DC Comics.
You proably could ost a few drawings and others would know what was involved here... I run a comic business and Frazetta cannot be beat and I imagine the the Brothers H. are second for me. I enjoy Kubert, but he is outclassed by Frazetta in a big way.
I don't know his name because they didn't credit him (or at least the copy I saw didn't give it), but he made some wonderful color illustrations for the original printing of ERB's books in the 1926 printing.