This example is one of three mythological landscape paintings from the Villa at Boscotrecase, discovered in the Bay of Naples and dating to the last decade of the first century BCE. It depicts the story of the hero Perseus saving Andromeda from the Ketos, a sea monster to which she had been sacrificed. The sea, sky and shore here all merge into one mystical landscape, where a few key features are picked out of the otherwise strange and indefinite background.
This image is in the Public Domain, courtesy of Wikimedia Commons.