THE MAD KING - Frank Frazetta for the ACE Books edition of 1964
About Wolheim, Krenkel, and Frazetta's start in paperback illustration:  

Donald Wolheim, the editor of ACE Books in the late '50s through the sixties, hired Roy G. Krenkel (AKA
RGK) to be the lead artist for the 1962-5 revival of Edgar Rice Burroughs’ works.  Wolheim had seen
Krenkel's work in fanzines such as
Amra, a fan produced magazine devoted to the works of Burroughs
and Conan creator Robert E. Howard.  Wolheim chose him, because to the ACE editor, Krenkel's style
resembled that of J. Allen St. John, the illustrator of many Burroughs first editions and magazine covers.   
The ACE editior pressured Krenkel to imitate St. John.  However, Krenkel's style simply couldn't be
sublimated.  RGK's clear and fluid line drawings and paintings are distinctively original and have become
iconic in the world of Burroughs illustration, particularly to baby-boomers discovering his work for the first
time in the early sixties.

Roy's first Edgar Rice Burroughs covers for ACE were for
At The Earth's Core (F-156 SF 1962) and The
Moon Maid (F-157 SF 1962).  Both were instant successes with the buying public, establishing a visual
style for future ACE editions and for ERB paperback and hardback reprints that publishers can't
completely escape to this day.  For an example, take a look at the Fall River Press editions of Burroughs
in book stores at this moment and you will see Roy's influence guiding the art direction of those editions.

Krenkel could create some of the most fluid drawings ever seen, but he was the first to admit that color
gave him problems.  Also, Roy wasn’t particularly good at meeting deadlines; so, he sometimes had his
friend Frank Frazetta, who was out of work at the time, help him with color and overall design.  He also
recommended Frazetta to Wolheim, but Wolheim was at first reluctant to use Frazetta's work.  When
Krenkel got far enough behind and couldn’t meet a deadline, Wolheim was left with little choice but to try
Frazetta out.   On short notice, Frazetta produced the iconic cover painting for the ACE edition of
Tarzan
and the Lost Empire.
The rest is history, because Frank Frazetta went on to become the most famous and
influential fantasy artist of the twentieth or any other century.
THE MAD KING - Frank Frazetta variant for G&D editions       $10.99
Using ACE illustrations for variants - Is it legal?:

The short answer is yes, because, to save costs, ACE never copyrighted their cover illustrations. During
the early 1960's, when Ace Books reprinted many Edgar Rice Burroughs books in the small sized 4 x 6.25
paperback, none of those books were copyrighted and all of the artwork became Public Domain
immediately. I don't use many of them for variants, but this is one of my favorites, and I couldn't resist
using this Frazetta on a DJ. This jacket also gave me a chance to say something positive about one of my
favorite adventure genres, the Ruritanian Romance.