Guidelines for writing a Chapter Book?

Those things can vary depending on your audience. What is you PB about? What is its word count? And maybe most importantly, what made them say that it should be a chapter book?

It's going to depend on the overall length of the book. There are a number of chapter books that end up in the "easy" section at the library -- the Nate the Great series, Henry and Mudge, a few of the Barker Twins books. They're all laid out very differently, but each has got a picture on every single page. You might try looking at the library or bookstore to get a feel for what's out there already, and that may give you a better idea on how to proceed.