Mural is a stone with carvings on it. Usually they have arrows and treasure chests(what a coincidence?!)
Have you ever noticed a flat square or triangular stone with carving? This is mural.

Murals are hadcrafted by island creator(he manually creates the drawings and glyphs on it).
Many island authors actually make simplistic signs or maps using these, which can really help you out with fining Loot chests(golden ones). It is pretty obvious, but many new players overlook that.