Authors and Illustrators

Once picture books are written, is the writer completely out of the picture?  Do the writers have a voice regarding the illustrations?

So, I have submitted picture books to agents, and one day, one of them will be in the hands of an illustrator (like my optimism here?).  Will the agent ask me my opinions?  Will they ask me if I think anything should be changed?  Don't get me wrong, I have a lot of faith in illustrators, and I believe they know how to make details shine through their artwork, but what if I think they are missing key details?

Is that silly?  After all, I only write the words, but I have no clue what the rules are.  Do you?