In order to insert images in the header section of your chapters, you'll first have to create a Custom theme. This tutorial will help with that: https://www.atticus.io/create-a-custom-theme/
While you're creating your custom theme, you can choose to either have all the header images the same, in which case you will upload the image in the theme settings once, and it will automatically show in all the chapters within your Body content.
If you'd like different images for each chapter, you will click "Use individual chapter images" and then for each chapter, use the Add Chapter Heading Image link near the chapter title in each individual chapter