Richard Richmond is convinced that this is a Morris Gollub painting. The glint on the blade is quite typical of Gollub's work. Compare Tarzan 57, Tarzan 64, and Indian Chief 12. All of these covers make use of a glint on the blade of knife or spear. The foreshortening of the water buffalo and the overall style and composition is consistent with the artwork of Moe Gollub.
ERBzine.com (http://www.erbzine.com/mag15/1576.html) also identifies this cover as a Gollub painting.