Your portfolio is your calling card. It tells a prospective publisher everything they need to know if they are to invest in your work. It's easier for a publisher to know who you are and what you stand for if you can show them at a glance that:
you understand picture books
you know your target audience
you have a consistent style
you are competent at maintaining character
you understand how to tell a story in pictures
That's why planning your portfolio is so important. It helps you get it right first time, allows you to identify areas where more work is needed, and makes it easy to replace old samples with updates as your work improves.
Follow my 6-part portfolio checklist to put together a portfolio that creates a memorable first impression.