How do I add a chapter screen?

I'm trying to create a new chapter screen for DSotSC that includes a unique graphic when the expansion is introduced. I have the text working but the graphic shows an existing image. I'm trying to understand how panels in GUICHAP.CHU are triggered when a new chapter starts. What file is responsible for this? Does anyone know of a mod that adds an addition panel to GUICHAP.CHU that I can look at for reference.