Unlike Galastel I'm not going to tell you that you can do picture books for adults because there are comic books for adults and medieval narrative poems with nineteenth century illustrations. What one has to do with the other, I don't know.
The problem you perceive and I see is that picture books are (today) mostly for children. There really isn't much of a market for adult picture books, and booksellers won't know where to put them in their stores. This is a serious obstacle, if you want to sell your book. I worked in a publishing house, and every year the sales representatives who had to visit the book stores and convince the booksellers to put our books in their shelves vetoed certain publications because they would fall between categories and bookstores wouldn't take them. The marked is a bit different today, what with many books sold from the websites of famous authors, but if you are a new author and don't have a large following on some social media site, you may want to think about how you are going to get people to know of your book.
That said, there is quite a number of picture books for adults. The link is to a Google search that comes up with a bunch of lists. Go through them and see what others have published, and do some research on how they market it.
If you don't care about sales, the answer to your question, of course, is: You can write anything you please.
As for Dinotopia, it is less a story than worldbuilding. Gurney is an illustrator, not a writer, and he had the idea for Dinotopia when he was recreating cities from archaeological finds for National Geographic. In an interview, he explains that his intention with Dinotopia was worldbuilding, and the story only served to frame the illustrations, in the way a travel journal ties together the photos of exotic places, but doesn't really narrate much besides that the places where visited and what went on there:
After painting a few of my "lost empires," I started playing with the idea of a picture book that would serve as a kind of grand tour. Then I thought of putting all those places on a single island, and populating the island with dinosaurs and people. I tried to immerse myself in every aspect of worldbuilding, from maps to mechanics to metaphysics, making it as real and believable as possible. I tried not to think of myself as creating the world, but instead just transcribing some lost journal. This attitude really freed me up. It's easier to transcribe something that already exists rather than creating something from whole cloth.
This travel-journal aspect of Dinotopia is the reason why the story, as you note, has no antagonist: it isn't a story in the way the writer of a novel thinks about story at all.
