The Main Difference
According to "Children̵7;s Literature, Briefly," picture books are a format of children̵7;s literature and picture storybooks are a category within that format.
Picture Books
Picture books include all different types of books where illustrations and text work together to tell the story or teach content. Picture books can be designed for a wide age range of readers, not just children.
Other Picture Book Categories
Alphabet books, counting books, wordless picture books, beginning reader books, and baby/board books are all other examples of types of picture books.
Picture Storybooks
The basic definition of a picture storybook is any picture book where the text and pictures work together to tell a story. They are the most popular and plentiful of the picture books. Many of the popular bedtime stories fall into this category.
Examples of Picture Storybooks
The first picture storybook published was Beatrix Potter's The Tale of Peter Rabbit in 1902. Since then, many other excellent books have followed. Some of the more well-known picture storybooks include Von Allsburg's "The Polar Express" and Sendak's "Where the Wild Things Are."