I don’t pick the illustrator for my books. The publisher does. They may ask for my opinion about the work of illustrators they’re considering, but the decision is ultimately up to them.
What if you don’t like the pictures?
I’ve been lucky to have illustrators who have brought wonderful visual ideas to the books, ideas that I would never have thought of.
I’ve also had the chance to see rough illustrations and have sometimes suggested changes that the illustrator was usually only too happy to make. Not all publishers give a writer this opportunity.