I recently submitted a proposal for a nonfiction picture book that I'm doing with a professional photographer. I sent in the proposal and she sent in a few sample photographs (not the ones that would eventually be used in the book, but just a few of our subject that will give the editor an idea of what the photographs could do for the text - and what she can do as a photographer). If you are mostly a writer, why not send in your query/proposal/ms. with a line that says you also have photographs that could be provided, if needed. That way, you're leaving the door open but your project won't be judged on whether your photographs are professional or not.
Some children's magazines do want writers to contribute photographs. So if the book project doesn't pan out, you might try writing a magazine story about one of your animals. Your photographs, in that case, might give you an advantage.