My sustained inability to find this visual gag is getting weird. Spoilered because it’s a bit horrific to type out (CW: animal cruelty), but trust me, it was hilarious:
A large ship like the Titanic is running dangerously low on fuel, so the men are ordered to shovel in more puppies. The puppies are shown crowded onto the shovel blade, all panting adorably, then are thrown into the furnace which makes an enormous WHOMPH and the ship gets moving again.
I’d figured it had to be the Titanic episode of Futurama (“A Flight To Remember”) but it seems not. Must have been one of those fleeting mental images from the Simpsons which are unconnected to the main plot, maybe?
ETA: as luck would have it – found it immediately after posting, apologies. It’s from The Canine Mutiny, the Simpsons episode where Bart gets the credit card in the name of Santos L. Halper. My difficulty finding this was searching for “puppies” where it was “shovel on more dogs.” not puppies.