Probably Burroughs or possibly Otis A. Kline. I do know that the bottom two are endpapers in the RGK Father of Heroic Fantasy tome. Krenkel often did studies of the same subject from different angles and many of them did not actually see print until these art collections.
You mentioned another book on my want-list, but yeah, Kline's another good possibility
We go broke because we know too much, but you might want to bookmark this for whenever; it's a pretty good deal...https://www.instocktrades.com/products/jan190862/art-of-roy-g-krenkel-father-of-heroic-fantasy-hc
Heck of a deal! I'll tell the kiddos (they've given up on shopping for me, so they just wait to hear what gifts I want for Christmas, birthdays, father's day, etc. etc.)