Before advice, I offer sympathy. I know how you feel, trust me. But since you're seeking suggestions, the best I can give you is this: enlighten your principal (or whoever holds the purse strings in your school). Some principals were not teachers, and those who were may never have set foot into a classroom below the middle or high school level. Some administrators are also unfamiliar with the scope and diversity of picture books available today. You mention picture books to them, and they're thinking Clifford the Big Red Dog. (Not that I'm knocking Clifford; he is old school cool).
What administrators most need help understanding, however, is the role picture books can play in the upper grades. Are they simply an add-on, or will they truly enrich and advance the curriculum we're presently teaching?
The best answer to the question, "Why Picture Books?" is a resource I created called Picture Books Across the Curriculum. There you'll find fifteen reasons for using picture books, complete with citations and additional reference sites. You and your colleagues may be able to brainstorm even more.
I would recommend that you invite administrators to your room when using picture books; show them that nothing beats a picture book for establishing a common literary culture. Prior to reading the novel Number the Stars, for example, I share several picture books about World War II, the Holocaust, and Denmark's resistance to the Nazis to provide my students with sufficient age-appropriate background knowledge. Otherwise, they simply could not understand, nor fully appreciate, the novel's events and themes. See my Holocaust Picture Books post for more titles and teaching ideas.