Going to steal this from McKee. An IMAGE SYSTEM in film is:
a strategy of motifs, a category of imagery embedded in the film that repeats in sight and sound from beginning to end with persistence and great variation, but with equally great subtlety, as a subliminal communication to increase the depth and complexity of aesthetic emotion
In other words, a director can create a series of visuals and/or sound effects that subliminally communicate the film's conflict towards the audience. He cites some examples, such as the water imagery in Les Diabolique and the prison imagery in Casablanca.
Les Diabolique: A boarding school master's wife and mistress conspire to kill him because he turns into a sadistic SOB. They get him drunk one night and together they drown him in the bathtub. They then dump his body in the school's outdoor swimming pool, which has been covered with algae all winter. They expect the body will float back to the surface the next day and be discovered, but a few days go by and nothing happens. One of the women drops her keys into the pool on purpose so they can order the pool to be drained, and the camera fixes on the water level as it goes down and down all the way ... to the drain. Nothing. Soon it's a mystery what on earth happened to the man.
The entire film it's drizzly and foggy. The camera occasionally focuses in on raindrops pattering windows, or condensation forming. When dinner is served, they eat fish. When the students and teachers talk about summer vacation, they talk about "taking to the waters". The title card is painted in abstract grays and a truck tire splashing a puddle is the next shot. Why is water so ubiquitous in this film? Just for the hell of it? So the director can say how artistic he is? Hell no.
In the climax, the wife fears her husband is a ghost and haunting her. She wakes up from a nightmare in the dead of night, unable to sleep, and suddenly she hears noises coming from the bathroom. Drip. Drip. Drip. It's water. She goes into the bathroom and the bathtub is overflowing with water. Something is in it. And then the frightening end to the movie.
Water was used as an image system of fear in Les Diabolique.
Casablanca: An image system of imprisonment is created to add to the sense that the protagonists are trapped in the city. The beacon on the airport tower revolves around the city like the search light of a prison yard. Blinds, room dividers, stair railings, even the leaves of potted palms are filmed in a way to create shadows that mimic prison bars. The characters constantly use the word "escape".
Of course the main plot in Casablanca is that everyone is trying to get letters of transit so they can leave the city and reach freedom in the west. The image system subliminally stresses the point of feeling trapped and trying to get out.
When it's a failure:
When a hack fills their film with image systems that have nothing to do with the plot. As RLM pointed out, there is a scene in The Empire Strikes Back when Leia shoots at Slave I from a landing platform on Bespin. She's trying to save Han, but is too late, and it's probably the last time she will ever see the man she loves.
Flash back to Attack of the Clones. Padme rushes onto a landing platform and fires her blaster at another ship that is taking off. It's Count Dooku trying to escape. There's no emotional connection there, it's just a good person firing a weapon at a bad person who gets away. Big f***ing deal.
In Return of the Jedi, Luke kicks Vader into a backflip during their emotional father/son duel.
In Revenge of the Sith, Anakin kicks Dooku, just some standard bad guy, into a backflip during their fight.
These "rhymes" don't serve any purpose to building the depth and complexity of aesthetic emotion. They are not an example of a strategic image system, they are an example of someone who ran out of creative ideas, and decided to just lift entire scenes from his previous films. In fact, if you actually watched the films in order 1-6 and managed to make the connection between those scenes, the effect would be to actually lessen the emotion you feel while watching. Imagine Luke kicks Vader down the stairs, and instead of focusing on their emotional conflict, you suddenly remember "oh, that was just like the time Anakin was fighting Dooku. Or imagine you see Leia heartbroken on the landing platform, and instead of connecting with her emotions, you are left thinking, "hey that's just like the time Padme was shooting at Dooku."
That's why the "it's like poetry, it rhymes" line invites so much derision.