The story
From poster + -ize (suffix forming verbs denoting something which is done or made). Sense 1.1 (“to reduce the number of colours”) refers to the original use of the process to produce posters. Sense 1.2 (“to score a slam dunk in basketball against (one or more opposing players) by leaping over them”) refers to the action being so spectacular as to warrant a photograph of it being printed on a poster.
Etymology from Wiktionary, CC BY-SA 4.0.