Lovecraft and others used the notion of highly detailed and descriptive murals in the ruins of ancient civilizations. Just by looking at what is painted or carved on the walls, the protagonists of a story can figure out how a ruin's ancient dwellers lived there lives. The "The Nameless City," the explorers see murals of reptilian beings going about all the city's business. While they are initially believed to be metaphorical, the lizard beings turn out to be real. Similarly, in "At the Mountains of Madness," the Elder Things history is detailed in bas-reliefs throughout the city.
Other authors from the pulp era used this trope as well, notably Clark Ashton Smith, as noted here.
What I'm wondering is in what work this trope originated. Or, if a particular work cannot be identified, what period this idea started appearing. If I had to venture a wild guess, I might think that it had something to do with the popularization of Egyptology in the late nineteenth or early twentieth centuries. Egyptian tombs were found to be extensively decorated with pictures of both mythical and mundane scenes.