It wasn’t me. I wasn’t there.
Without signage, it’s hard for me to think of a purely visual gag that really worked on the Simpsons. So much of the humor was in the writing, dialog, and situations.
It was Olmec. Ollll-mec.
–Cliffy, 
*Homer is bothering a film crew person, and the film guy tells Homer that there are donuts in the trailer. Before he can finish his sentence, we see Homer zip away but the Homer-shaped dust from his body still stands there.
*The Raffi-like singer is giving a concert, and a bunch of babies are passing around a pacifier like it’s a joint.
*When Homer becomes a hippie, the two hairs on his head have a hip little curl in them.
D’oh!! Maybe we can make peace… I was misunderstanding your point. Thought you were specifically trying to say that they grew from molecules into cells, not just that their outline seemed to change.
Shrug. “It happens” 
The only one I can think of that hasn’t been mentioned yet: The Halloween episode where the Simpsons are abducted by Kang and Kodos. Each family member is zipped up into the saucer by a tractor beam. But when they try to lift Homer, he only gets a couple of feet off the ground before the saucer starts straining and faltering. Then they shine two more beams on him and he finally gets lifted. And of course he continues to munch his hamburger the whole. 
I LOVE this gag. I know it’s a visual only thread, but the little yelp he gives at the same time compeltes it all the more.
The episode where Homer wants to get into shape.
He gets up early one morning, goes out front and starts joging on the sidewalk, then he is walking, then crawling. When he can go no further he looks up and is in front of Flander’s mailbox.
Same episode where Homer is climbing the Murderhorn with the Sherpas. He pisses them off, and they leave him on the mountain. Marge and the kids are in the house discussing Homer’s chances, and someone (Marge?) says that he will be OK because of the Sherpas. Through the window we see the Sherpas hitchhiking with a sign that says “Tibet”, a pickup stops the sherpas get in and drive away.
I thought of this one today:
When Homer and Flanders are temporary best friends, both families go on a picnic. During a waterskiing adventure, Flanders, driving the powerboat, asks Homer how Rod and Todd are doing. Homer absentmindedly says “They’re fine.” The camera cuts to the boys’ tow lines skipping accross the water.
My favorite is at the apple orchard, when Homer is talking to Flanders. “If it’s clear and yella’, you’ve got juice there, fella! If it’s tangy and brown, you’re in cider town!” You just hear Homer’s mind say “You can stick around, but I’m outta here!” A second later, Homer, with a dumb look on his, just falls over.
I laugh my ass off every time.
My favorites all revolve around the kids, not sure why.
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Bart Sells His Soul - Desperate to retrieve his soul, Bart kneels by his bed and prays for help. Sure enough, the ‘receipt’ for his soul comes floating down. He looks up and we see Lisa standing over him, standing on tiptoe with her arm in the air. I love that pose! Even more: seconds later, the image of Bart hugging the piece of paper. Incredibly cute.
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Bart vs. Thanksgiving - Well, I love the whole scene where Bart asks Marge to help with dinner preparations (and does almost nothing but ask Marge for help). But the visual-only part that gets me is when Bart holds an open can of cranberry sauce over a bowl as he waits for the gelatinous stuff to fall; when it does, it’s still can-shaped for a few seconds before it collapses with a “plop.” I have no idea why this makes me giggle, but it does. (Of course, it’s capped when Bart proudly proclaims this recipe “Cranberry Sauce a la Bart!”)
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Bart on the Road - Now this is the one that I immediately thought of when I saw this thread. Bart, Milhouse, Martin and Nelson are in Branson (I think?) watching Hank Williams sing “Moon River.” We watch the slow pan over the bored faces of Bart, Milhouse & Martin … only to end on Nelson’s face, frozen in an expression of enrapt delight. Seeing thuggy Nelson so uncharacteristically blissful never fails to crack my shit up.
And finally, although this may count as signage, I must include it.
- Lisa’s Substitute - Four words: You are Lisa Simpson. Sob!
Alternately, the scene where Bart tries to get Ralph’s soul and Chief Wiggum flashes a light in his eyes and they’re demon eyes.
In the episode where Marge and Ruth Powers do the Thelma & Louise thing there’s a shot near the end of Lionel Hutz babysitting Bart and Lisa. He’s sitting on the couch with on leg propped up on the other, eating a popsicle. He’s dressed in sweat pants and a t-shirt, the look on his face is priceless.
Along the same lines: the very first time Groundskeeper Willie ripped his shirt off, revealing his hyper-muscular physique. I think the first time it happened was in the “Bart falls down a well” episode. It was so unexpected, given Willie’s normal outward appearance, that I choked with laughter.
In one case, Lisa, Bart and Homer are in the living room watching TV. Either Bart or Homer complains about something on *Itchy & Scratchy * being unrealistic. Lisa says something to the effect of, “But it’s a cartoon, it doesn’t have to be realistic” as Homer is seen passing the window outside (while he’s still sitting on the couch). That always cracked me up.
I think that was probably Andy Williams.
Unless it was a Halloween episode.
Another one I thought of: the shadow burned into the wall where Homer used to sit in front of the Radiation King TV in his childhood home.
In the episode where Milhouse ran away from the movie set, and hid in the tree house – how many corners did that tree house have?
(hey, I laughed…)
There was an episode where homer pondered what fictional depictions of bart and lisa would look like, and in his imagination they are basically realistic depictions of Bart and Lisa. Homer shudders
I loved the saucer-spinning gag in the anti-kid episode. Marge organizes a kid-friendly lobby or something at this restaurant. She says, “We’ll need a little bit of money…” and when the camera pans out the whole restaurant is devoid of people and there is this saucer spinning (as if someone ran so fast they made it spin). Marge sighs, and Luigi asks, “About the check…” Marge says,“hey look over there!” and when Luigi turns back to marge, she is gone, and the sacuer is spinning
Luigi sighs, then 2 goverment guys show up and say, “We’re from the INS…” and Luigi is gone, and there is a saucer spinning once more 
The Bible stories in church episode: It’s a hot day and the church’s air conditioning is broken. Homer has put a chocolate bunny in the collection plate ("Hey, I found it in the dumpster). Rev. Lovejoy puts it on the pulpit. Flanders says it’s hard to hear the reverend with the fans blowing, so he turns them off.
Every time the Rev. is seen starting a different story, the chocolate bunny is seen slowly melting into a puddle of chocolate. Great sight gag!
To his shovel: “Come on Agnes. We have work to do.”
Another gag in “Homer the Smithers” is when Homer is telling Mr. Burns about his [Burns] impounded car. Mr. Burns is getting more and more annoyed and has this great look on his face.