Googling "hero image" yields the answer in less time than writing a comment on HN.
I can't find a satisfactory answer. Wikipedia doesn't say. [1] says it comes from the print industry, but I can't find any indication that that's true (all the search results seem to be related to online, "hero image print industry" doesn't turn up anything useful). "Hero image etymology" isn't helpful either.
[1] http://line25.com/articles/30-web-designs-that-fully-embrace...
Anyway, "In theater and film, a hero prop (sometimes just "hero") is a prop that is intended to be seen close up so it's a lot more detailed than others. That's the oldest definition I know.". The source is in the reply to the sibling comment.
https://www.designernews.co/stories/47337-why-is-the-hero-im...