Where the Wild Things Are

I truly enjoyed Where the Wild Things Are. I am incredibly intrigued by art and love pen and ink type illustrations, so these really appeal to me. The story may be simple, but the illustrations say so much about Max's imagination, and that of Maurice Sendak as well. Art is a passion of mine and I marvel at those who can not only create one picture that tells a story, but a series and then put just the right words with those pictures. Which comes first for this writer, the pictures or the story? How do most author/illustrator types go about composing what they write and draw?