The illustration is meant to be symbolic, but if one squints and uses a little bit of imagination, one can imagine this illustration as depicting a wizard or a priest of the Old Ones opening a portal to the Outer Spheres, allowing the Hounds of Tindalos to emerge, while in the background Yog Sothoth rages in the sky (as per the climax of the Dunwich Horror) and the Old Ones smite a city, as in the passage from the Necronomicon quoted in Dunwich Horror: "They bend the forest and crush the city..." The ship tossing in the sea must therefore represent the rising of Cthulhu from "The Call of Cthulhu." Surely, this literal interpretation of what we see in the picture makes more intuitive sense than claiming it a symbolic illustration of the consequences of Adam and Eve's sex life!
The point, of course, is that when looking at weird old art, one can see what one wants to see. The question is whether that interpretation can be supported by facts, evidence, logic, and reasoning. This picture is obviously not a picture of the rising of Lovecraft's Old Ones, nor has it ever been shown that any ancient art is pictures of aliens.