There's not a Wikipedia page for "visual metaphor", but there are pages for "visual rhetoric" https://en.wikipedia.org/wiki/Visual_rhetoric and "visual thinking" https://en.wikipedia.org/wiki/Visual_thinking

Negative space can be both meaningful and useful later on.

I learned about visual thinking and visual metaphor in application to business communications from "The Back of the Napkin: Solving Problems and Selling Ideas with Pictures" http://www.danroam.com/the-back-of-the-napkin/