Hold on, how is "hero" a food metaphor? I mean, I understand that there are some regional dialects that use that name for a "submarine" sandwich (and there are many other names for it), but I can't fathom how a full-screen image at the top of a website has any metaphorical connection to that. To me, that makes even less sense than the idea that such an image somehow is supposed to do a heroic job of advocating for whatever is the main point of the page (unironically my prior mental model!).
It's not a food metaphor, nor is it about displaying something heroic, although that's much closer. "Hero image" comes from "hero props" which "are the more detailed pieces intended for close inspection by the camera or audience. ... The name refers to their typical use by main characters in a production." [0]
Since the name arises from use by a hero, then to extend the metaphor by direct analogy, the actual hero is the overall article/content in which a hero image is contained.
That said, a "hero sandwich" is that which "one needed to be a hero to finish" [1] so does all tie back to the idea of heroism regardless.
