The idea being expressed in a poster for 20,000 leagues under the sea is:
What is the idea of a picture?
The idea of a picture is the main point that its author wants the viewer to see. The persons and things in the picture can be translated and used to form impressions.
When the photographer captures some leagues under the sea, he probably wants to imprint the fear of monsters in viewers.
Learn more about science fiction here:
https://brainly.com/question/1675278
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