Um, I wanted to do this a very long time, since I began making comics.
Make an ad, in the setting, that disrupts the flow of the narrative, for the sake of an ad unrelated to what is discussed or happening in the story. I remember marvel and dc would sell ad space for like GI Joes or Xray specs or red rider air rifles.
I chose to paint my mother as isis holding a can of nile ale. In ancient egypt, the setting of my work, cold beer and mead was desired, as sometimes the nile would flood and be unpotable, and because cool beverages were a treat. Mom requested a green and blue eye.
It had me jokingly thinking of selling ad space for portraits of people for the purpose of advertising fake products, but products and services found in the setting of a chapter.