I have made two attempts at writing a picture book. The first time, the editor I submitted it to said the humor was more appropriate for middle grade students, suggested I rewrite and resubmit it. That became my first book, My Life Among the Aliens. When I tried again, a writing group partner suggested that effort would work better as a chapter book. My editor agreed with her. That evolved into A Girl, a Boy, and a Monster Cat.
My take away from these two experiences is that not every idea is appropriate for a picture book. Unfortunately, I've got nothing on what exactly is a workable picture book idea.
I have another take away on picture books from a teacher's conference I attended in 1999. Cecilia Yung explained that the pictures in picture books don't just illustrate text. They actually carry part of the story themselves. Things like setting, characters' emotions, some action don't appear in the text. They appear in the illustration. A reader takes in the whole story at once through text and image. The illustrations in a picture book can even have their own storyline.
This was kind of mind boggling to me. It's one thing for author/illustrators to create a picture because they can work both aspects of the story at the same time. But how do writers working on their own create a story that doesn't include large amounts of the information that goes into the illustrations but isn't so bare bones that agents and editors don't find it uninteresting?
Clearly, I've never been able to work that out.