Two of my favorites:
A bunch of T-Rex dinosaurs standing around smoking surreptitiously. Caption: The real reason dinosaurs became extinct.
A bunch of lemmings about to jump of a cliff, but one is wearing a flotation device and smiling.
Beat me to it! My Mother once gave me a coffee mug with that one, a not-so-subtle attempt to get me to quit smoking.
My favorite: A dog riding in a car, excitedly leaning out the passenger-side window, tail wagging, says to a bunch of other dogs standing on the corner, “Hey guys! Guess what? I’m going to the vets to get TUTORED!”
Distant hills
The distant hills call to me
Their rolling waves seduce my heart
Oh, how I want to graze in their lush valleys
Oh, how I want to run down their green slopes
Alas I cannot
Damn the electric fence!
Damn the electric fence!
"Embedded in styrofoam shoes, Carl is sent to ‘Sleep with the humans’ "
And
“Going rogue in his later years Dumbo became a major hazard for airliners”
Another of my favorites, which I had forgotten to mention:
Caveman sitting atop a huge device, looking through a hollowed-out log at a mammoth below, saying “It’s a mammoth.” The caption: “Early microscope”
A bunch of cavement squinting at petri dishes. The caption: “Early microbiologists”
A caveman making a film presentation to an audience of other cavemen, pointing at the spiky tail of a creature. Caption: “This part is called the Thagomizer, after the late Thag Simmons.”
A bunch of large, round footprints, one of them containing a flattened caveman with a piece of hollow bamboo in his mouth. Caption: “1,000,000 BC: Gak Eisenberg invents the first, and last, silent mammoth whistle.”
That man was FUNNY.
Bummer of a birthmark, Hal.
Four goldfish in a fishbowl, two of them are floating. The female goldfish says to her husband, “I told you, but no, you just had to give the Finkelsteins seconds. And now look at them. They’re floating!”
First pants, THEN shoes.
What, the thread gets this far and no mention of “Cow Tools” yet? 
2 alien youths have an astronaut in a glass jar, there’s 2 versions on this theme…
1; “Dont forget to poke holes in the lid this time”
2; 2 astronauts in the jar, one of the alien kids says “Cool, shake the jar and see if they’ll fight”
a pair of frogs are in a car, the male frog is at the wheel with a blank expression, the female frog is berating his driving…
“look out for that tree, now we’re over the line, criminy, you’re driving like you’ve been pithed or something…”
a man and a woman in a car…on the moon
“Harold, look where the Earth is, pull over and let me drive, we’re lost”
Two snakes, one male and the other female. The male is looking in a mirror with a concerned look on his face. He has a little puffy ring around his middle. His wife says “Don’t worry, honey, it’s just love handles.”
Two devils are talking to each other in an office in hell. One asks the other while looking oudside a window at some miserable looking slaves “Roy…am I a butthead?”.
There are penguins as far as the eye can see, and they all look exactly the same. In the middle one is standing ontop of something and singing “I got to be me, oh I just gotta be me!”
The second one was actually printed out as a poster, but the penguin that is standing out is painted yellow. This heinous change disturbed Larson because he thought it might make some people think he was being sentimenal. Not to mention because whoever printed the poster out totally didn’t get the joke
Don’t have time to read through the whole thread, but the one where it says something like “What dogs think”…
and there’s an ecstatic-looking dog thinking “Tomorrow I get to poop again!”
Scene: A cliff overlooking an expanse of water…with a slick of fire on the water.
Caption: When cliffdivers belly-flop.
A couple walks into their house to see their young child, sweating and fatigued, pinning a grand piano to the ceiling using a broomstick handle.
Several small aliens (from the movie Aliens) are sitting around a table. In the middle of the table is a roast turkey with an alien bursting out of it. One of the aliens is yelling “Mom! Billy’s playing with his food again!”
A woman is walking her dog, a man is walking his alligator. The dog is inside the alligator’s mouth. The woman says “Oh no! Your alligator ate my dog!” And the man is thinking “Man, this is such a great way to meet women!”
Two dogs on a date, sitting on the couch, and the male is covertly spraying his mouth with a can of Doggie Breath.
A watched head is never eaten by ants.
One of my favourite things about Larson was the names his characters got. They make every cartoon funnier:
Two bears in the woods. One says “Is the pope Polish? Does a bear - well, I know YOU do, Angelo.”
Two children sitting on the living room floor watching TV, Mom sitting in a chair looking back at Dad, who is coming up behind the children, while holding a HUGE insect-looking beast, saying to Mom “Ssssh, I wanna surprise the kids.”
A couple of medival soldiers up on a wall, arrows whizzing by…hitting others.the uniforms have bulls-eyes on them. One has soldier turned to another, “I told you these new uniforms were a crock!”
According to my palentology professor, it’s really called that now. 
My favorite is the “Thag, wall of ice closer today?”
I would need to pull out my copy to be sure, but I believe Jane Goodall even wrote the foreword for Pre-History.
Another favorite:
I like the one where detectives are surveying a crime scene in a clock shop. They have cause of death and identity of the victim figured out, and all they need, as the chief detective says, is to nail down a time of death.
Every single clock hanging on the walls of the shop has been broken, and each has the same time displayed on its face.