The undisputed king of naïve painters, Henri Rousseau was, for the first half of his career, a toll collector who only painted in his spare time. He came to some prominence - and more ridicule - in his own time with his series of jungle paintings, the first of them being this one:
Many art critics of the day saw an inexperienced, childlike painting, but others, including none other than Pablo Picasso, championed Rousseau.
Recognizing a Rousseau is easy. Look for "flat," vividly colored, imagined jungle landscapes. Rousseau himself had never been to a jungle - he is said to have never left France! - so he cobbled together animals he saw at the Paris Zoo and plants he saw at the Jardin des Plants, making his paintings something of "jungle capricci," with flora and fauna that do not go together in any natural context. It's probably silly to use a term such as capriccio in the context of Rousseau though, given that he does not seem to have been much of a student of art history. Since he did not formally study under anyone, his paintings don't resemble some other master's. In fact, while one could argue that the flatness of his paintings and bold blocks of color seem to borrow from his contemporary Jean-Léon Gérôme and his imagined landscapes and anthropomorphic animals invite some comparisons to the great animal painter George Stubbs, it's not clear that Rousseau truly emulated anyone's style in particular. He is a painter unto his own.
Another jungle scene:
And a jungle-y portrait: