Chapter 3
The Marauders learnt early on that the best way to get away with their pranks was to get the ghosts and portraits to work with them. The Marauders spent weeks listening to the portrait's life stories until they almost dropped dead from boredom. All of this was worth it when the Marauders managed to execute a perfect prank on Slytherin by having all of the paintings whisper to each other when the Slytherins walked past until the Slytherins became so paranoid that they were constantly watching the paintings. Meanwhile the ghosts disrupted the house elves enough that the Marauders could slip a newly brewed potion, that would cause warts to sprout on the Slytherins bodies, into the Slytherin's food.
This resulted in a hilarious sight at dinner when suddenly shrieks erupted from the Slytherin table. The Marauders could not contain their glee but with the loud laughter from the portraits and ghosts the Slytherins could not figure out who to blame. Surely the portraits and ghosts could not have done this to them, right?
The ghosts and portraits were always happy to help the Marauders after that.