Deep fog to convey ominousness
At several points in the film, fog is used to set an intimidating tone; one example of this comes as Buttercup and her kidnappers sail down the foggy river at night, with a mysterious second boat in tow, capturing a mood of uncertainty and unease. A second comes during the Fire Swamp, which is thick with trees, vine, and yes, fog, to convey how on-edge we and our heroes should feel about being inside it.
Count Rugen's wounds at the film's climax
The image of Count Rugen pleading for his life with bleeding, identical cuts on his cheeks strikes an important parallel image to the wounds he inflicted on Inigo. In fact, Inigo makes a point of delivering the exact some number of wounds to Rugen as he did to him, and in the same places: two on the shoulders, two on his face, and finally the one through his stomach. The parallel imagery of their wounds serves to illustrate that Rugen's death is entirely a matter of revenge.
The Cliffs of Insanity
The Cliffs of Insanity are particularly memorable for being sheer and foreboding. Filmmakers used the Cliffs of Moher in Ireland as the backdrop for this scene, providing an huge physical obstacle for the man in black, later revealed to be Westley, to overcome. Not only do they stand as a visually impressive backdrop for him to give chase, but also a solid metaphor for the lengths he's willing to go to to save Buttercup.
Buttercup falling into Fezzik's arms
Near the film's end, Buttercup jumps from her castle window into Fezzik's arms. The scene is shot such that she seems almost to float as she descends, as if in control of her movements, perhaps even flying. This could be symbolic of her liberation from her betrothal to Humperdinck and freedom to escape the castle and be with Westley. It's the first time in the film since she became known as the princess bride that she's truly free, and the image of her free-falling to safety reflects that.
