There are picture books for adults in the sense that:
-they're meant for children, but they're good for adults, too. (Ex: Shaun Tan's Lost and Found)
-they look like they're for kids, but they're actually for adults because they're too inappropriate. (Ex: Adam Mansbach's Go the F**k to Sleep)
-they're graphic novels or comic books.
I dislike the notion that 'pictures are for kids'.
What about picture books for adults because...they're meant for adults? Like the way you wouldn't give a book like Passage to India to a 6-year-old (that is, it's for adults because of its sensibilities and complexity of understanding, rather than obscenity). I'm talking about picture books as a (hypothical) genre that is as broad as the fiction/fantasy/sci-fi/historical/romance/classic/etc. genres. I'm talking about books with the format of a children's picture book (creatively integrated text, illustration/graphic design-heavy, generally non-sequential) combined with the complexity of both fine art and a novel like Nathaniel Hawthorne's The Scarlet Letter. (Not talking about graphic novels, though. No comic sans font, bolded words for emphasis, panels of sequential art... Not that genre.)
It could be like a children's book, except without the usual constraints (does it have any inappropriate themes, does it educate or inspire in a way that is socially acceptable, are the words simple enough, are there too many pages...).
Imagine this conversation in a parallel universe where there is no public notion of 'pictures are for kids':
"Hey, have you heard about _______ [title of some hypothetical adult picture book]? It's been getting a lot of reviews. Haven't read it myself yet. I'm still on ______ [title of another adult picture book]."
"Yyyes, I have. Kind of. I read it quickly on the first read, then went back and ended up spending, like, fifteen minutes on each page, trying to analyze the picture, then the text, then the picture and the text in relation to each other, then the symbols, and the overarching themes...yeah. I'm still kind of thinking about it. I gotta say, this one's a doozy for me. At one point, (you don't mind mild spoilers, right? Right.) there's this squint-or-you'll-miss-it bit of text in a specific part of the illustration that might or might not have been text, or even intended at all, and if you take it seriously it changes the entire book. Seriously. I was looking from my tarot deck to the book to the tarot deck again and I was like...'what is the difference.'"
"Wow. I have a friend who beat you, though--"
"--The attic guy who spends his whole time in the attic on one book at a time?"
"Yes. Stop calling him that. Anyway, he's got you beat. I told you he usually goes through books really fast, right? I talked to him last Tuesday, and he said he took the whole month this time to get through and move on from ________."
(Note: Conversation I made up off the top of my head; I hope you don't mind that I'm expounding unnecessarily on this. I've been thinking about this idly for awhile now, and there are just so many scenarios and possibilities bouncing around in my head...)
And what I think would be the cherry on top: writer and illustrator finding each other organically, working together through the whole process like two close collaborators, rather than being uncommunicative and match-made by a publishing company; the picture book equivalent of today's indie game developers.
I expect that there are already people out there who are two steps ahead of me and already published such books, but I have no idea who they are or whether they are many or few. Am I living under a rock and they've been there this whole time? Is there a demand for this kind of picture book? Would it be a foolish pursuit to produce such a book? How unrealistic is any of what I've said above? Am I missing something? Because right now I wish this actually existed and I could walk to Barnes & Noble and find that half the store is picture books, divided into sub-genres.