If you develop your stories with text, write freely first, then each time you revisit your story craft the maximum meaning into fewer and fewer words. If you prefer to work visually, sketch as much as possible until the images start to pick up some of the slack. The reason they are called picture books is because the words and pictures are equally important. More than that they complement each other and together create a new language.
Let’s break that down; Some people write on their own, some illustrate, some do both. Publishers can start the process from either point but tend to work with words first more often. They will read the text and then make a decision about the illustrator based on their company’s style.
Writers, your part of the book project is going to have other hands and brains joining in. It might be a good idea to write with that in mind.