In 1963, Robert K. Abbett was commissioned by Ballantine Books to paint the covers for Edgar Rice Burroughs’ Barsoom series. 1965 saw the addition of John Carter of Mars as the 11th book, and Thuvia received a new cover in 1969. Abbett would also paint a Tarzan series of covers.
For the two different editions of Thuvia, I’ve read two stories. The first is that Ballantine accidentally published one of Abbett’s roughs (Thuvia with the banth), and they didn’t correct it until later. The second story is that John Carter helping Phaidor out of the flier was too risqué for 1963, what with the skimpy bikini, and going with the more chaste mini-skirt was the compromise until society caught up. I’m sure someone more knowledgable than myself will bring some clarity to this.
Abbett was one of the premier illustrators of his day, eventually leaving the commercial art game to master another field, painting dogs for the print and gallery market.
I haven’t been able to gather a complete set of virgin art to include here yet, but I did find a few of them.