The subject of the image is one or more creatures of the sea, earth or sky. Man made objects may be part of the image while any human presence in the image should be incidental or non-existent.
So an image of a crow in a birdbath would be acceptable (man made object, no human presence) while an image of a man walking four dogs would not. An acceptable example of human presence would be an image of a breaching orca whale with a person in a kayak the background. If the person in the kayak was in the foreground of the scene about to drenched with water, then the image would not meet the incidental requirement of human presence.