The same rule applies to visual metaphors: Just as any
literary metaphor, a visual metaphor confuses if it doesn’t
clarify; it breaks if you stretch it; and it becomes
ridiculous if you combine it with a second or third
metaphor.
Avoid metaphor that do not clarify; if a metaphor darkens
the meaning of what you are trying to express, don’t use it.
I think that it would be clearer if there were some examples of nested metaphors or metaphors that 'darken' other metaphors. Maybe there is in the actual text, but the quote out of context seems a little abstract.
