And if it doesn't strictly have to be a painting then how about a tapestry with lots of little images woven into it's pattern?
Weaveworld
Weaveworld - Wikipedia
en.m.wikipedia.org
Never read it myself, I did see a review once that pointed out similarities to Weaveworld@dannymcg I never read Guy Gavriel Kay's Fionovar Tapestry series, but aren't all the people already in the tapestry and only become aware of itthrough experience?
I'm surprised no one's mentioned The Neverending Story. The novel and film features a boy the world of a book.
Not sure why, but that reminds me of a few episodes of Supernatural, in particular one in which Sam and Dean go though some kind of rift in space/time and find themselves as actors in a show called Supernatural, and Sam's name here is Jerod Padalecki and his actress wife was the demon, Ruby, in the "real" world.
Was that firs the one with the Trickster where they ended up in a CSI: Miami type situation, whipping off multiple sunglasses ala David Caruso?