Robert Hays taking a drink and missing his mouth in Airplane
Gene Kelly dancing around the lamppost in Singing in the Rain
Julie Andrews opening her umbrella and flying away in Mary Poppins
Indianna Jones grabbing his hat before the door comes down.
The shark’s head coming up over the side of the boat in Jaws.
John Travolta striking his disco pose in Saturday Night Fever.
The helicopters coming up over the beach in Apocalypse Now.
I was going to say the shot of Devil’s Tower in Close Encounters, but then I started thinking that any picture of Devil’s Tower brings Close Encounters to mind for many people.
I seem to be having a very 1970s moment.
Dustin Hoffman with Anne Bancroft’s leg in the foreground in The Graduate.
Charlie Chaplin caught in the machinery in Modern Times.
Marlon Brando screaming “Stella! Hey, Stella!” in A Streetcar Named Desire.
The home run smashing the lights in The Natural.
Sylvester Stallone on the top of the library steps in Rocky.
In no particular order:
The British Redcoats unleashing a volley of rifle-fire on an impi of attacking Zulu, from (unsurprisingly) Zulu
Neo dodging the bullets from an Agent’s gun in Bullet Time, from The Matrix
Indiana Jones running from the boulder (although Indy on horseback firing a Webley revolver comes very close!) from Raiders Of The Lost Ark (and Indiana Jones & The Last Crusade)
The shot of Lawrence crossing endless sand-dunes from Lawrence of Arabia
The final shoot-out from The Wild Bunch
Edward Fox as The Jackal, taking aim at Charles DeGaulle in The Day Of The Jackal
Any lightsabre duel from the original Star Wars trilogy
The duel with the Black Knight from Monty Python & The Holy Grail
Black Bart (Cleavon Little) arrives in Rock Ridge, from Blazing Saddles
“The First Rule of Fight Club…”, from Fight Club
Far more iconic is the Knight playing chess with Death, which has been endlessly referenced and parodied.
Some more:
The Cowboy aiming his gun at the audience and firing (Great Train Robbery)
Peter Lore’s reflection in the candy store window behind the little girl (M)
The Uboat crew Singing “Tiparary” (Das Boot)
Lawrence of Arabia has quite a few The train coming off the rails and into the camera. Lawrence standing in his new white robes. Sharrif Ali’s entrance. The end of the first act as the General and politician walk away obviously aware they will undermine Lawrence.
The monster’s first entrance (Frankenstien)
I second lots that have been posted. I did read the thread, so hopefully I don’t duplicate too many, but I’ll add:
Colonel Saito smacking Colonel Nicholson with the Geneva Convention pamphlet, and the British troops getting ready to riot; from The Bridge on the River Kwai. You could also add any shot of the titular bridge, specifically it being destroyed and “What have I done?”
Paul Newman and Jackie Gleason playing pool; The Hustler
Mel Gibson and Catherine McCormack holding hands in profile at their wedding; Braveheart
Gary Cooper giving Lou Gherig’s famous speech; The Pride of the Yankees
“You’re gonna need a bigger boat” and the shark eating Quint; Jaws
The “Black” Sox walking out of the cornfield; Field of Dreams
Al Pacino firing the machine gun (“Say hallo to my little frien”); Scarface
Roberto Benigni comically goosestepping with the Nazis for his son’s sake; Life is Beautiful
Any landmark being destroyed; *Independence Day *
Sharon Stone sitting in the interrogation chair; Basic Instinct
The Earps and Doc Holliday walking side-by-side down the main street; Tombstone
Ralph Macchio at the final tourmament, in profile preparing the crane kick; The Karate Kid. Alternately, same shot on the piling at the beach.
I agree with this one for Star Wars. I don’t think there’s any one moment that your average moviegoer could picture in their head clearer than this one.
War is Hell moments:
Russian Roulette in The Deer Hunter
The last moments of Gallipoli
Horror moments:
The taking of the ear in Reservoir Dogs
Hannibal Lecter coldly staring from in his cell
Butch and Sundance jumping off the cliff into the river…“ooooohhhh shiiiiiiiiiiiiiiiiiiiiiiitttttttttt”
And Thelma and Louise going off the cliff.
Sorry, Happy Clam, I just saw your Lechter in a cell entry.
And Bo Derek emerging from the waves in 10.
How about that “Top of the World, Ma!” guy?
James Cagney in ‘White Heat’. *Good * choice.
…and the baby carriage rattling down the steps in The Untouchables. 
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The edit from the ape-tossed bone to the space shuttle moving to the station in 2001. Clearly one of the most important seconds in film history, the edit itself traverses millions of years in between two frames of film and is enormously powerful.
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The little boy in Sixth Sense saying " I see dead people".
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Jack Lemmon laying on the floor, shot in the gut in a nuclear power plant in " The China Syndrome", whispering " I can feeeeeeeel it. "
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Spock saying, " For everything there is a first time".
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Jack Nicholson’s chicken salad speech from " Five Easy Pieces".
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The last sixty seconds of " The Bad Seed".
Cartooniverse
And how about his noodle salad speech in As Good As It Gets? That guy’s all about the casual lunchtime dining.
But seriously, Bonnie and Clyde posing for gangster photos, and getting shot up at the end.
Sally Field holding up the Strike sign in Norma Rae.
“Attica Attica!” in Dog Day Afternoon.
“Snap out of it” in Moonstruck.
The kids in the spoonful of Cheerios in Honey I Shrunk the Kids.
“You talkin to me??” in Taxi Driver.
The speeding car in Back to the Future.
Sandy as the slut at the end of Grease.
:smack: Or, more correctly, the Union sign. It really is iconic, I swear.
For film images of real events, how about:
The moon landing.
The atom bomb.
The liberation of the Nazi death camps.
The poppies in Flanders’ fields.
For the unreal, how about Andy Serkis’s soliloquy in The Two Towers?
The end of ‘Butch Cassidy and the Sundance Kid’