My original tumblr post was here. This image was originally published by gmgallery with a bleg as to its origins. The site notpulpcovers responded at length:
It is by Malcolm Smith, and it was published in Other Worlds Issue #1, November 1949, to illustrate the story The Fall Of Lemuria.
That complete issue can be downloaded here.
The whole post comes to us via doppelgaenger18.
Ah yes, Richard Shaver, the Tolstoy of the late pulp era!